diff --git a/crates/clawhdf5-bench/src/bin/read_harness.rs b/crates/clawhdf5-bench/src/bin/read_harness.rs
index b09879a..0a517d6 100644
--- a/crates/clawhdf5-bench/src/bin/read_harness.rs
+++ b/crates/clawhdf5-bench/src/bin/read_harness.rs
@@ -7,15 +7,19 @@
 //! ```text
 //! cargo run --release -p clawhdf5-bench --bin read_harness
 //! cargo run --release -p clawhdf5-bench --bin read_harness -- --large   # 512 MB
+//! cargo run --release -p clawhdf5-bench --bin read_harness -- --v18      # HDF5 1.8 format
+//! cargo run --release -p clawhdf5-bench --bin read_harness -- --chunk 32 # 32 x 32 chunks
 //! ```
+//!
+//! `--v18` writes the file with `libver_bounds(V18, V18)` (version-1 B-tree
+//! chunk indexes) instead of the default 1.10 format (Fixed Array indexes
+//! here), to compare the two.
 
 use std::time::{Duration, Instant};
 
-use clawhdf5::{File, FileBuilder};
+use clawhdf5::{File, FileBuilder, LibVer};
 use clawhdf5_format::selection::Selection;
 
-const CHUNK: u64 = 256;
-
 struct Layout {
     name: &'static str,
     chunked: bool,
@@ -46,16 +50,19 @@ fn value(row: u64, col: u64) -> f64 {
     (row * 100_003 + col) as f64 * 0.5
 }
 
-fn write_file(path: &std::path::Path, rows: u64, cols: u64) {
+fn write_file(path: &std::path::Path, rows: u64, cols: u64, chunk: u64, v18: bool) {
     let data: Vec<f64> = (0..rows)
         .flat_map(|r| (0..cols).map(move |c| value(r, c)))
         .collect();
     let mut builder = FileBuilder::new();
+    if v18 {
+        builder.libver_bounds(LibVer::V18, LibVer::V18);
+    }
     for (i, layout) in LAYOUTS.iter().enumerate() {
         let ds = builder.create_dataset(&format!("d{i}"));
         ds.with_f64_data(&data).with_shape(&[rows, cols]);
         if layout.chunked {
-            ds.with_chunks(&[CHUNK, CHUNK]);
+            ds.with_chunks(&[chunk, chunk]);
         }
         if layout.deflate {
             ds.with_deflate(4);
@@ -91,7 +98,14 @@ fn slab(start: [u64; 2], count: [u64; 2]) -> Selection {
 }
 
 fn main() {
-    let large = std::env::args().any(|a| a == "--large");
+    let args: Vec<String> = std::env::args().collect();
+    let large = args.iter().any(|a| a == "--large");
+    let v18 = args.iter().any(|a| a == "--v18");
+    let chunk: u64 = args
+        .iter()
+        .position(|a| a == "--chunk")
+        .and_then(|i| args.get(i + 1))
+        .map_or(256, |c| c.parse().expect("--chunk N"));
     let (rows, cols) = if large { (8192, 8192) } else { (4096, 2048) };
     let total_mb = (rows * cols * 8) as f64 / (1 << 20) as f64;
     if cfg!(debug_assertions) {
@@ -100,12 +114,21 @@ fn main() {
 
     let dir = tempfile::TempDir::new().unwrap();
     let path = dir.path().join("read_harness.h5");
-    write_file(&path, rows, cols);
-    let file_mb = std::fs::metadata(&path).unwrap().len() as f64 / (1 << 20) as f64;
+    let t = Instant::now();
+    write_file(&path, rows, cols, chunk, v18);
+    let write_ms = t.elapsed().as_secs_f64() * 1e3;
+    let file_bytes = std::fs::metadata(&path).unwrap().len();
+    let file_mb = file_bytes as f64 / (1 << 20) as f64;
 
     println!("## Read harness");
     println!(
-        "\n{rows} x {cols} f64 ({total_mb:.0} MB per dataset), chunks {CHUNK} x {CHUNK}, file {file_mb:.0} MB\n"
+        "\n{rows} x {cols} f64 ({total_mb:.0} MB per dataset), chunks {chunk} x {chunk}, \
+         format {}, file {file_mb:.0} MB ({file_bytes} bytes), written in {write_ms:.0} ms\n",
+        if v18 {
+            "1.8 (v1 B-tree)"
+        } else {
+            "1.10 (default)"
+        }
     );
 
     // (label, selection, elements selected)
diff --git a/crates/clawhdf5-format/src/btree_v1_write.rs b/crates/clawhdf5-format/src/btree_v1_write.rs
new file mode 100644
index 0000000..c0524d8
--- /dev/null
+++ b/crates/clawhdf5-format/src/btree_v1_write.rs
@@ -0,0 +1,470 @@
+//! Writing a version-1 B-tree chunk index (node type 1): the chunk index of
+//! layout message versions 1-3, and the only one HDF5 1.8 reads.
+//!
+//! The tree is built the way libhdf5 builds it when the chunks reach it one
+//! after another in row-major order (a whole-dataset `H5Dwrite` of a 1-D
+//! dataset, or of any dataset without a chunk cache; with one, libhdf5
+//! inserts the small chunks of a multi-dimensional dataset in the order its
+//! cache evicts them, which fills the nodes differently): each
+//! chunk goes through the same steps as `H5B_insert` (`H5B.c`) with the
+//! chunk callbacks of `H5Dbtree.c`, so nodes split where libhdf5's split,
+//! with its default split ratios (a full right-most node keeps 90% of its
+//! children, a left-most one 10%, any other half), and keys hold what
+//! libhdf5's hold:
+//!
+//! - a chunk's key is its size in the file, its filter mask and its offsets
+//!   (the element-size coordinate 0);
+//! - a node's final key is the zero-size key one chunk past the chunk that
+//!   last moved it (every scaled coordinate plus one, `H5D__btree_new_node`),
+//!   which libhdf5 moves only when a new chunk is not below it
+//!   (`H5D__btree_cmp3`) — so after an even number of appends in one
+//!   dimension it lies on the last chunk itself;
+//! - a full root is copied to a new node and becomes the parent of the copy
+//!   and its new sibling, so the root's address (the layout message's) never
+//!   changes.
+//!
+//! Nodes are laid out in the order libhdf5 allocates them (the root first,
+//! then each new node as a split creates it), all of the full node size, the
+//! unused slots zero.
+
+#[cfg(not(feature = "std"))]
+use alloc::{format, vec, vec::Vec};
+
+use core::cmp::Ordering;
+
+use crate::error::FormatError;
+
+/// libhdf5's default chunk B-tree K (`HDF5_BTREE_CHUNK_IK_DEF`): nodes hold
+/// up to 2K = 64 children. Superblocks of version 2 cannot record another
+/// value without a superblock extension, which this writer does not emit.
+pub(crate) const CHUNK_BTREE_K: u16 = 32;
+
+/// libhdf5's default split ratios (`H5D_XFER_BTREE_SPLIT_RATIO_DEF`) for a
+/// left-most, middle and right-most node.
+const SPLIT_RATIOS: [f64; 3] = [0.1, 0.5, 0.9];
+
+/// A chunk to index: scaled coordinates (offset / chunk dimension) in each
+/// dataset dimension, stored size, filter mask and address.
+pub(crate) struct ChunkEntry {
+    pub(crate) scaled: Vec<u64>,
+    pub(crate) nbytes: u64,
+    pub(crate) filter_mask: u32,
+    pub(crate) address: u64,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct Key {
+    nbytes: u32,
+    mask: u32,
+    /// Scaled coordinates, the element-size one (0 or 1) last.
+    scaled: Vec<u64>,
+}
+
+impl Key {
+    /// `H5D__btree_new_node`'s right key: one chunk past `self` in every
+    /// dimension, with no storage.
+    fn right_of(&self) -> Key {
+        Key {
+            nbytes: 0,
+            mask: 0,
+            scaled: self.scaled.iter().map(|s| s + 1).collect(),
+        }
+    }
+
+    fn cmp_scaled(&self, other: &Key) -> Ordering {
+        self.scaled.cmp(&other.scaled)
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Node {
+    level: u8,
+    left: Option<usize>,
+    right: Option<usize>,
+    /// `children.len() + 1` keys once the node holds a child.
+    keys: Vec<Key>,
+    /// Chunk addresses in a leaf, node indexes above.
+    children: Vec<u64>,
+}
+
+/// What an insertion below a node did (`H5B__insert_helper`'s outputs).
+#[derive(Default)]
+struct Ret {
+    /// The node's new left key (`lt_key_changed`).
+    lt: Option<Key>,
+    /// The node's new right key (`rt_key_changed`).
+    rt: Option<Key>,
+    /// The node split: the key shared by the halves and the new right node.
+    split: Option<(Key, usize)>,
+}
+
+struct Tree {
+    nodes: Vec<Node>,
+    two_k: usize,
+}
+
+fn bad(why: &str) -> FormatError {
+    FormatError::SerializationError(format!("version-1 B-tree chunk index: {why}"))
+}
+
+impl Tree {
+    fn new(k: u16) -> Self {
+        Self {
+            nodes: vec![Node {
+                level: 0,
+                left: None,
+                right: None,
+                keys: Vec::new(),
+                children: Vec::new(),
+            }],
+            two_k: 2 * usize::from(k),
+        }
+    }
+
+    /// `H5B_insert` of `key` (a chunk after every chunk already inserted).
+    fn insert(&mut self, key: &Key, addr: u64) -> Result<(), FormatError> {
+        let r = self.insert_helper(0, key, addr, 64)?;
+        let Some((md, split)) = r.split else {
+            return Ok(());
+        };
+        // The root split: copy it to a new node and make the root the
+        // parent of the copy and its new right sibling.
+        let lt = r.lt.unwrap_or_else(|| self.nodes[0].keys[0].clone());
+        let rt = match r.rt {
+            Some(rt) => rt,
+            None => self.nodes[split]
+                .keys
+                .last()
+                .cloned()
+                .ok_or_else(|| bad("empty node"))?,
+        };
+        let moved = self.nodes[0].clone();
+        let level = moved.level;
+        let moved_id = self.nodes.len();
+        self.nodes.push(moved);
+        self.nodes[split].left = Some(moved_id);
+        self.nodes[0] = Node {
+            level: level + 1,
+            left: None,
+            right: None,
+            keys: vec![lt, md, rt],
+            children: vec![moved_id as u64, split as u64],
+        };
+        Ok(())
+    }
+
+    fn insert_helper(
+        &mut self,
+        id: usize,
+        key: &Key,
+        addr: u64,
+        depth: u8,
+    ) -> Result<Ret, FormatError> {
+        if depth == 0 {
+            return Err(bad("tree too deep"));
+        }
+        let n = self.nodes[id].children.len();
+        let level = self.nodes[id].level;
+        let mut ret = Ret::default();
+        if n == 0 {
+            // The first chunk (H5B_INS_FIRST): its key and the right key.
+            let node = &mut self.nodes[id];
+            node.keys = vec![key.clone(), key.right_of()];
+            node.children = vec![addr];
+            return Ok(ret);
+        }
+        // Binary search with H5D__btree_cmp3: 1 when the chunk is not below
+        // the right key, -1 when below the left key, else 0.
+        let (mut lo, mut hi, mut idx) = (0usize, n, 0usize);
+        let mut cmp = Ordering::Less;
+        while lo < hi && cmp != Ordering::Equal {
+            idx = (lo + hi) / 2;
+            let node = &self.nodes[id];
+            cmp = if key.cmp_scaled(&node.keys[idx + 1]) != Ordering::Less {
+                Ordering::Greater
+            } else if key.cmp_scaled(&node.keys[idx]) == Ordering::Less {
+                Ordering::Less
+            } else {
+                Ordering::Equal
+            };
+            if cmp == Ordering::Less {
+                hi = idx;
+            } else {
+                lo = idx + 1;
+            }
+        }
+        let (mut lt_changed, mut rt_changed) = (false, false);
+        // The child to add after child `idx`, with its left key.
+        let mut new_child: Option<(Key, u64)> = None;
+        match cmp {
+            Ordering::Less => return Err(bad("chunks out of order")),
+            Ordering::Greater if idx + 1 < n => {
+                return Err(bad("cannot place chunk"));
+            }
+            Ordering::Greater if level == 0 => {
+                // Past every chunk of the right-most leaf: a new maximum
+                // (H5B_INS_RIGHT through `new_node`), which moves the right
+                // key one chunk past it.
+                idx = n - 1;
+                self.nodes[id].keys[idx + 1] = key.right_of();
+                rt_changed = true;
+                new_child = Some((key.clone(), addr));
+            }
+            Ordering::Equal if level == 0 => {
+                // Inside the last chunk's range: H5D__btree_insert adds it
+                // to the right of that chunk; the right key stays.
+                if key.scaled == self.nodes[id].keys[idx].scaled {
+                    return Err(bad("duplicate chunk"));
+                }
+                new_child = Some((key.clone(), addr));
+            }
+            _ => {
+                if cmp == Ordering::Greater {
+                    idx = n - 1;
+                }
+                let child = usize::try_from(self.nodes[id].children[idx])
+                    .map_err(|_| bad("bad node index"))?;
+                let r = self.insert_helper(child, key, addr, depth - 1)?;
+                if let Some(lt) = r.lt {
+                    self.nodes[id].keys[idx] = lt;
+                    lt_changed = true;
+                }
+                if let Some(rt) = r.rt {
+                    self.nodes[id].keys[idx + 1] = rt;
+                    rt_changed = true;
+                }
+                if let Some((md, split)) = r.split {
+                    new_child = Some((md, split as u64));
+                }
+            }
+        }
+        // Pass the node's changed end keys up, as H5B__insert_helper does.
+        if lt_changed && idx == 0 {
+            ret.lt = Some(self.nodes[id].keys[0].clone());
+        }
+        if rt_changed && idx + 1 >= n {
+            ret.rt = Some(self.nodes[id].keys[idx + 1].clone());
+        }
+        if let Some((md, child)) = new_child {
+            // A full node splits first; the child goes to the half that
+            // holds child `idx`.
+            let (mut target, mut split) = (id, None);
+            if n == self.two_k {
+                let s = self.split(id, idx);
+                let nleft = self.nodes[id].children.len();
+                if idx >= nleft {
+                    idx -= nleft;
+                    target = s;
+                }
+                split = Some(s);
+            }
+            // H5B__insert_child (H5B_INS_RIGHT): the new child after child
+            // `idx`, its left key after that child's.
+            let node = &mut self.nodes[target];
+            node.keys.insert(idx + 1, md);
+            node.children.insert(idx + 1, child);
+            ret.split = split.map(|s| (self.nodes[s].keys[0].clone(), s));
+        }
+        Ok(ret)
+    }
+
+    /// `H5B__split` of the full node `id`, the insertion going after child
+    /// `idx`; returns the new right node.
+    fn split(&mut self, id: usize, idx: usize) -> usize {
+        let node = &self.nodes[id];
+        let ratio = if node.right.is_none() {
+            SPLIT_RATIOS[2]
+        } else if node.left.is_none() {
+            SPLIT_RATIOS[0]
+        } else {
+            SPLIT_RATIOS[1]
+        };
+        let mut nleft = (self.two_k as f64 * ratio) as usize;
+        if idx < nleft && nleft == self.two_k {
+            nleft -= 1;
+        } else if idx >= nleft && nleft == 0 {
+            nleft += 1;
+        }
+        let new_id = self.nodes.len();
+        let right = Node {
+            level: node.level,
+            left: Some(id),
+            right: node.right,
+            keys: node.keys[nleft..].to_vec(),
+            children: node.children[nleft..].to_vec(),
+        };
+        let old_right = node.right;
+        self.nodes.push(right);
+        if let Some(r) = old_right {
+            self.nodes[r].left = Some(new_id);
+        }
+        let node = &mut self.nodes[id];
+        node.keys.truncate(nleft + 1);
+        node.children.truncate(nleft);
+        node.right = Some(new_id);
+        new_id
+    }
+}
+
+/// Bytes of one node of a chunk B-tree with `ndims` key dimensions (the
+/// dataset's rank plus the element-size one).
+fn node_size(two_k: usize, ndims: usize, offset_size: usize) -> usize {
+    let key = 8 + 8 * ndims;
+    8 + 2 * offset_size + (two_k + 1) * key + two_k * offset_size
+}
+
+/// Build the chunk B-tree for `chunks`, given in row-major order of their
+/// scaled coordinates, with nodes laid out from `base_address`. `chunk_dims`
+/// are the chunk's dimensions (the dataset's rank of them) and `elem_size`
+/// the element size, the key's last dimension. Returns the nodes' bytes; the
+/// root is at `base_address`. `chunks` must not be empty: an index without
+/// chunks has no tree (its address is undefined).
+pub(crate) fn build_chunk_btree_v1_at(
+    chunks: &[ChunkEntry],
+    chunk_dims: &[u64],
+    elem_size: u32,
+    base_address: u64,
+    offset_size: u8,
+) -> Result<Vec<u8>, FormatError> {
+    if chunks.is_empty() {
+        return Err(bad("no chunks"));
+    }
+    let rank = chunk_dims.len();
+    let mut tree = Tree::new(CHUNK_BTREE_K);
+    for c in chunks {
+        if c.scaled.len() != rank {
+            return Err(bad("chunk rank differs from the dataset's"));
+        }
+        let nbytes = u32::try_from(c.nbytes).map_err(|_| {
+            FormatError::SerializationError(format!(
+                "a chunk of {} bytes cannot be indexed by a version-1 B-tree \
+                 (HDF5 1.8 chunks are under 4 GiB)",
+                c.nbytes
+            ))
+        })?;
+        let mut scaled = c.scaled.clone();
+        scaled.push(0);
+        let key = Key {
+            nbytes,
+            mask: c.filter_mask,
+            scaled,
+        };
+        tree.insert(&key, c.address)?;
+    }
+
+    let os = usize::from(offset_size);
+    let ndims = rank + 1;
+    let nsize = node_size(tree.two_k, ndims, os);
+    let addr_of = |id: usize| base_address + (id * nsize) as u64;
+    let mut dims: Vec<u64> = chunk_dims.to_vec();
+    dims.push(u64::from(elem_size));
+    let mut out = vec![0u8; tree.nodes.len() * nsize];
+    for (i, node) in tree.nodes.iter().enumerate() {
+        let d = &mut out[i * nsize..(i + 1) * nsize];
+        d[0..4].copy_from_slice(b"TREE");
+        d[4] = 1; // node type: raw data chunks
+        d[5] = node.level;
+        let n = u16::try_from(node.children.len()).map_err(|_| bad("node too large"))?;
+        d[6..8].copy_from_slice(&n.to_le_bytes());
+        let undef = u64::MAX;
+        put_addr(&mut d[8..], node.left.map_or(undef, addr_of), os);
+        put_addr(&mut d[8 + os..], node.right.map_or(undef, addr_of), os);
+        let mut p = 8 + 2 * os;
+        for (k, key) in node.keys.iter().enumerate() {
+            d[p..p + 4].copy_from_slice(&key.nbytes.to_le_bytes());
+            d[p + 4..p + 8].copy_from_slice(&key.mask.to_le_bytes());
+            for (j, (&s, &dim)) in key.scaled.iter().zip(&dims).enumerate() {
+                let off = s
+                    .checked_mul(dim)
+                    .ok_or_else(|| FormatError::Overflow("chunk key offset".into()))?;
+                d[p + 8 + 8 * j..p + 16 + 8 * j].copy_from_slice(&off.to_le_bytes());
+            }
+            p += 8 + 8 * ndims;
+            if let Some(&child) = node.children.get(k) {
+                let a = if node.level == 0 {
+                    child
+                } else {
+                    addr_of(usize::try_from(child).map_err(|_| bad("bad node index"))?)
+                };
+                put_addr(&mut d[p..], a, os);
+                p += os;
+            }
+        }
+    }
+    Ok(out)
+}
+
+fn put_addr(d: &mut [u8], v: u64, os: usize) {
+    d[..os].copy_from_slice(&v.to_le_bytes()[..os]);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn build(n: u64) -> Tree {
+        let mut t = Tree::new(CHUNK_BTREE_K);
+        for i in 0..n {
+            let key = Key {
+                nbytes: 80,
+                mask: 0,
+                scaled: vec![i, 0],
+            };
+            t.insert(&key, 1000 + i).unwrap();
+        }
+        t
+    }
+
+    /// Leaves in order from the root, with their child counts.
+    fn leaves(t: &Tree, id: usize, out: &mut Vec<usize>) {
+        let n = &t.nodes[id];
+        if n.level == 0 {
+            out.push(n.children.len());
+        } else {
+            for &c in &n.children {
+                leaves(t, c as usize, out);
+            }
+        }
+    }
+
+    #[test]
+    fn sequential_appends_split_as_libhdf5_does() {
+        // libhdf5 2.0 (h5py, libver=('v108', 'latest')) writes 1000 chunks
+        // as a root over 17 leaves of 57 chunks and one of 31, with the
+        // root's right key on the last chunk (9990, 8 for 10-element f8
+        // chunks).
+        let t = build(1000);
+        assert_eq!(t.nodes[0].level, 1);
+        let mut l = Vec::new();
+        leaves(&t, 0, &mut l);
+        let mut want = vec![57; 17];
+        want.push(31);
+        assert_eq!(l, want);
+        assert_eq!(t.nodes[0].keys.last().unwrap().scaled, vec![999, 1]);
+        // 100 000 chunks: three levels, a root of 31 children.
+        let t = build(100_000);
+        assert_eq!(t.nodes[0].level, 2);
+        assert_eq!(t.nodes[0].children.len(), 31);
+    }
+
+    #[test]
+    fn right_key_moves_every_other_append() {
+        let t = build(5);
+        assert_eq!(t.nodes[0].keys.last().unwrap().scaled, vec![5, 1]);
+        let t = build(6);
+        assert_eq!(t.nodes[0].keys.last().unwrap().scaled, vec![5, 1]);
+    }
+
+    #[test]
+    fn keys_and_siblings_are_consistent() {
+        let t = build(5000);
+        for (i, n) in t.nodes.iter().enumerate() {
+            assert!(n.children.len() <= t.two_k);
+            assert_eq!(n.keys.len(), n.children.len() + 1);
+            if let Some(r) = n.right {
+                assert_eq!(t.nodes[r].left, Some(i));
+                assert_eq!(n.keys.last(), t.nodes[r].keys.first());
+            }
+        }
+    }
+}
diff --git a/crates/clawhdf5-format/src/chunked_write.rs b/crates/clawhdf5-format/src/chunked_write.rs
index 78c4161..10f1099 100644
--- a/crates/clawhdf5-format/src/chunked_write.rs
+++ b/crates/clawhdf5-format/src/chunked_write.rs
@@ -7,6 +7,7 @@ use crate::addr::saturating_usize;
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
+use crate::btree_v1_write;
 use crate::btree_v2_write::{BTreeV2Params, build_btree_v2};
 use crate::checksum::jenkins_lookup3;
 use crate::chunk_cache::{CACHE_LINE_SIZE, align_to_cache_line};
@@ -19,6 +20,7 @@ use crate::filter_pipeline::{
     FilterPipeline,
 };
 use crate::filters::compress_chunk_masked;
+use crate::libver::LibVer;
 /// Round a file offset up to the next cache-line boundary.
 ///
 /// This ensures chunk data starts at an address that is a multiple of the
@@ -866,6 +868,23 @@ pub fn build_chunked_data_from_precompressed(
     base_address: u64,
     maxshape: Option<&[u64]>,
 ) -> Result<ChunkedDataResult, FormatError> {
+    build_chunked_data_from_precompressed_libver(pre, base_address, maxshape, LibVer::Latest)
+}
+
+/// [`build_chunked_data_from_precompressed`] for a file whose low library
+/// version bound is `low`: below [`LibVer::V110`] (that is, for HDF5 1.8)
+/// every chunked dataset gets a version-3 layout message and a version-1
+/// B-tree chunk index, whatever its maximum shape, as libhdf5 writes it;
+/// otherwise the version-4 layout and the index libhdf5 picks for it.
+pub fn build_chunked_data_from_precompressed_libver(
+    pre: &PrecompressedChunks,
+    base_address: u64,
+    maxshape: Option<&[u64]>,
+    low: LibVer,
+) -> Result<ChunkedDataResult, FormatError> {
+    if low < LibVer::V110 {
+        return build_btree_v1_chunked_data(pre, base_address, maxshape);
+    }
     let index = ChunkIndexPlan::new(&pre.shape, maxshape, &pre.chunk_dims)?;
     let offset_size: u8 = 8;
     let length_size: u8 = 8;
@@ -992,6 +1011,92 @@ pub fn build_chunked_data_from_precompressed(
     })
 }
 
+/// Lay out precompressed chunks at `base_address` followed by a version-1
+/// B-tree chunk index, with a version-3 layout message: what libhdf5 writes
+/// for a chunked dataset under a low bound of 1.8.
+fn build_btree_v1_chunked_data(
+    pre: &PrecompressedChunks,
+    base_address: u64,
+    maxshape: Option<&[u64]>,
+) -> Result<ChunkedDataResult, FormatError> {
+    if let Some(ms) = maxshape {
+        let bad = |what: &str| FormatError::ChunkedReadError(format!("maxshape: {what}"));
+        if ms.len() != pre.shape.len() {
+            return Err(bad("rank differs from the shape"));
+        }
+        if ms.iter().zip(&pre.shape).any(|(&m, &s)| m < s) {
+            return Err(bad("smaller than the shape"));
+        }
+    }
+    let offset_size: u8 = 8;
+    let mut data_buf = Vec::new();
+    let mut entries = Vec::with_capacity(pre.chunks.len());
+    for (i, (_raw_size, stored, filter_mask)) in pre.chunks.iter().enumerate() {
+        let aligned_offset = align_to_cache_line(data_buf.len());
+        if aligned_offset > data_buf.len() {
+            data_buf.resize(aligned_offset, 0u8);
+        }
+        entries.push(btree_v1_write::ChunkEntry {
+            scaled: scaled_coords(&pre.shape, &pre.chunk_dims, i),
+            nbytes: stored.len() as u64,
+            filter_mask: *filter_mask,
+            address: base_address + data_buf.len() as u64,
+        });
+        data_buf.extend_from_slice(stored);
+    }
+    let element_size = u32::try_from(pre.element_size)
+        .map_err(|_| FormatError::Overflow("element size".into()))?;
+    // A dataset with no chunks has no tree: its address is undefined, as
+    // libhdf5 leaves it until the first chunk is written.
+    let btree_address = if entries.is_empty() {
+        u64::MAX
+    } else {
+        let aligned_idx = align_to_cache_line(data_buf.len());
+        if aligned_idx > data_buf.len() {
+            data_buf.resize(aligned_idx, 0u8);
+        }
+        let addr = base_address + data_buf.len() as u64;
+        let tree = btree_v1_write::build_chunk_btree_v1_at(
+            &entries,
+            &pre.chunk_dims,
+            element_size,
+            addr,
+            offset_size,
+        )?;
+        data_buf.extend_from_slice(&tree);
+        addr
+    };
+    let layout_message =
+        serialize_v3_chunked(&pre.chunk_dims, btree_address, offset_size, element_size)?;
+    Ok(ChunkedDataResult {
+        data_bytes: data_buf,
+        layout_message,
+        pipeline_message: pre.pipeline_message.clone(),
+    })
+}
+
+/// A version-3 layout message for a chunked dataset: dimensionality (the
+/// rank plus one), the B-tree's address, then each chunk dimension and the
+/// element size, four bytes each.
+fn serialize_v3_chunked(
+    chunk_dims: &[u64],
+    btree_address: u64,
+    offset_size: u8,
+    element_size: u32,
+) -> Result<Vec<u8>, FormatError> {
+    let ndims = u8::try_from(chunk_dims.len() + 1)
+        .map_err(|_| FormatError::Overflow("chunked layout rank".into()))?;
+    let mut buf = vec![3u8, 2, ndims];
+    push_addr(&mut buf, btree_address, offset_size);
+    for &d in chunk_dims {
+        let d =
+            u32::try_from(d).map_err(|_| FormatError::Overflow(format!("chunk dimension {d}")))?;
+        buf.extend_from_slice(&d.to_le_bytes());
+    }
+    buf.extend_from_slice(&element_size.to_le_bytes());
+    Ok(buf)
+}
+
 /// Most slots a Fixed Array index may have before we refuse to build it: its
 /// data block holds one element per chunk of the *maximum* extent, so a huge
 /// finite maxshape with small chunks would otherwise exhaust memory.
diff --git a/crates/clawhdf5-format/src/datatype.rs b/crates/clawhdf5-format/src/datatype.rs
index 8e2120d..5e9a678 100644
--- a/crates/clawhdf5-format/src/datatype.rs
+++ b/crates/clawhdf5-format/src/datatype.rs
@@ -1216,6 +1216,26 @@ impl Datatype {
         }
     }
 
+    /// The highest datatype message version in this type's encoding, its
+    /// members' and base types' included (the version decides which HDF5
+    /// releases can read it: 1-3 HDF5 1.8, 4 HDF5 1.12, 5 HDF5 2.0).
+    pub fn max_encoded_version(&self) -> u8 {
+        let own = self.serialize().first().map_or(0, |b| b >> 4);
+        let inner = match self {
+            Datatype::Compound { members, .. } => members
+                .iter()
+                .map(|m| m.datatype.max_encoded_version())
+                .max()
+                .unwrap_or(0),
+            Datatype::Enumeration { base_type, .. }
+            | Datatype::VariableLength { base_type, .. }
+            | Datatype::Array { base_type, .. }
+            | Datatype::Complex { base_type, .. } => base_type.max_encoded_version(),
+            _ => 0,
+        };
+        own.max(inner)
+    }
+
     /// Check that this datatype can be written: every part of it has an
     /// on-disk encoding, and the encoding is one the reader (and libhdf5)
     /// accepts. [`Self::serialize`] cannot report errors, so the writer calls
diff --git a/crates/clawhdf5-format/src/error.rs b/crates/clawhdf5-format/src/error.rs
index 449ff13..9dd22b4 100644
--- a/crates/clawhdf5-format/src/error.rs
+++ b/crates/clawhdf5-format/src/error.rs
@@ -167,6 +167,19 @@ pub enum FormatError {
     VlDataError(String),
     /// Serialization error.
     SerializationError(String),
+    /// The file's library version bounds
+    /// ([`FileWriter::libver_bounds`](crate::file_writer::FileWriter::libver_bounds))
+    /// do not allow what was asked for: `what` needs the format of HDF5
+    /// `needs` or later, and the high bound is `high` (or the low bound is
+    /// above the high one, with `needs` the low bound).
+    LibverBound {
+        /// What cannot be written.
+        what: String,
+        /// The oldest release whose format holds it.
+        needs: crate::libver::LibVer,
+        /// The file's high bound.
+        high: crate::libver::LibVer,
+    },
     /// Dataset is missing data.
     DatasetMissingData,
     /// Dataset is missing shape.
@@ -450,6 +463,13 @@ impl fmt::Display for FormatError {
             FormatError::SerializationError(msg) => {
                 write!(f, "serialization error: {msg}")
             }
+            FormatError::LibverBound { what, needs, high } => {
+                write!(
+                    f,
+                    "{what} needs the HDF5 {needs} file format, above the high \
+                     library version bound ({high})"
+                )
+            }
             FormatError::DatasetMissingData => {
                 write!(f, "dataset is missing data")
             }
diff --git a/crates/clawhdf5-format/src/file_writer.rs b/crates/clawhdf5-format/src/file_writer.rs
index c64e1cf..6dcfe47 100644
--- a/crates/clawhdf5-format/src/file_writer.rs
+++ b/crates/clawhdf5-format/src/file_writer.rs
@@ -5,12 +5,13 @@
 
 use crate::addr::saturating_usize;
 #[cfg(not(feature = "std"))]
-use alloc::{format, vec, vec::Vec};
+use alloc::{format, string::String, vec, vec::Vec};
 
 use crate::attribute::AttributeMessage;
 use crate::btree_v2_write::{BTreeV2Params, build_btree_v2};
 use crate::chunked_write::{
-    ChunkOptions, PrecompressedChunks, build_chunked_data_from_precompressed, precompress_chunks,
+    ChunkOptions, PrecompressedChunks, build_chunked_data_from_precompressed_libver,
+    precompress_chunks,
 };
 use crate::data_layout::VdsMapping;
 use crate::dataspace::{Dataspace, DataspaceType};
@@ -31,6 +32,7 @@ pub use crate::type_builders::ProvenanceConfig;
 pub use crate::type_builders::{AttrValue, CompoundTypeBuilder, EnumTypeBuilder};
 
 use crate::datatype::{CharacterSet, Datatype};
+use crate::libver::LibVer;
 
 pub(crate) const OFFSET_SIZE: u8 = 8;
 pub(crate) const LENGTH_SIZE: u8 = 8;
@@ -168,13 +170,15 @@ pub(crate) fn build_dataset_oh(
     attrs: AttrStorage<'_>,
     fill_message: &[u8],
     refcount: u32,
+    layout_version: u8,
 ) -> Result<Vec<u8>, FormatError> {
     let mut w = ObjectHeaderWriter::new();
     w.add_message_with_flags(MessageType::Datatype, dt.serialize(), 0x01);
     w.add_message(MessageType::Dataspace, ds.serialize(LENGTH_SIZE));
     w.add_message_with_flags(MessageType::FillValue, fill_message.to_vec(), 0x01);
+    // Versions 3 and 4 encode a contiguous layout the same way.
     let mut dl = Vec::new();
-    dl.push(4); // version
+    dl.push(layout_version);
     dl.push(1); // class = contiguous
     // An empty dataset has no storage: its address must be the undefined
     // address, as libhdf5 writes it. A real address with size 0 trips
@@ -198,14 +202,16 @@ pub(crate) fn build_compact_dataset_oh(
     attrs: AttrStorage<'_>,
     fill_message: &[u8],
     refcount: u32,
+    layout_version: u8,
 ) -> Result<Vec<u8>, FormatError> {
     let mut w = ObjectHeaderWriter::new();
     w.add_message_with_flags(MessageType::Datatype, dt.serialize(), 0x01);
     w.add_message(MessageType::Dataspace, ds.serialize(LENGTH_SIZE));
     w.add_message_with_flags(MessageType::FillValue, fill_message.to_vec(), 0x01);
-    // Compact layout message: version=4, class=0, u16 size, inline data
+    // Compact layout message: version (3 and 4 are the same here), class=0,
+    // u16 size, inline data
     let mut dl = Vec::new();
-    dl.push(4); // version
+    dl.push(layout_version);
     dl.push(0); // class = compact
     dl.extend_from_slice(&(data.len() as u16).to_le_bytes());
     dl.extend_from_slice(data);
@@ -1371,6 +1377,10 @@ pub struct FileWriter {
     /// file-space strategy (a File Space Info message in the superblock
     /// extension).
     page_size: Option<u32>,
+    /// Library version bounds: the low bound picks the format versions
+    /// written, the high bound limits the features allowed.
+    low: LibVer,
+    high: LibVer,
 }
 
 impl Default for FileWriter {
@@ -1485,9 +1495,37 @@ impl FileWriter {
             alignment_threshold: 0,
             alignment_bytes: 0,
             page_size: None,
+            low: LibVer::V110,
+            high: LibVer::Latest,
         }
     }
 
+    /// Set the library version bounds, as libhdf5's `H5Pset_libver_bounds`
+    /// (h5py's `libver=(low, high)`): the oldest HDF5 release whose format
+    /// the file uses (`low`), and the newest whose features it may use
+    /// (`high`). See [`crate::libver`] for what each bound changes.
+    ///
+    /// The default, `(LibVer::V110, LibVer::Latest)`, is what clawhdf5 has
+    /// always written: the HDF5 1.10 format (version-3 superblock, version-4
+    /// layouts with the 1.10 chunk indexes), readable by HDF5 1.10 and later.
+    ///
+    /// `(LibVer::V18, LibVer::V18)` writes a file HDF5 1.8 can read — the
+    /// low bound libhdf5 2.0 uses by default: a version-2 superblock,
+    /// version-3 layouts, and a version-1 B-tree for every chunked dataset,
+    /// resizable ones included; [`Self::finish`] then fails with
+    /// [`FormatError::LibverBound`] for anything HDF5 1.8 cannot read
+    /// (virtual datasets, a paged file, the 1.12 reference types, native
+    /// complex numbers). With a low bound of 1.8 and a later high bound
+    /// such objects are written in the newer format, as libhdf5 writes them;
+    /// the rest of the file stays readable by 1.8.
+    ///
+    /// A low bound above the high bound makes [`Self::finish`] fail.
+    pub fn libver_bounds(&mut self, low: LibVer, high: LibVer) -> &mut Self {
+        self.low = low;
+        self.high = high;
+        self
+    }
+
     /// Set global file alignment: datasets with raw data >= `threshold` bytes
     /// will have their data aligned to `bytes` boundary.
     ///
@@ -1583,6 +1621,33 @@ impl FileWriter {
             )));
         }
 
+        let (low, high) = (self.low, self.high);
+        let within_bounds = |what: &dyn Fn() -> String, needs: LibVer| {
+            if needs > high {
+                Err(FormatError::LibverBound {
+                    what: what(),
+                    needs,
+                    high,
+                })
+            } else {
+                Ok(())
+            }
+        };
+        within_bounds(&|| format!("a low library version bound of {low}"), low)?;
+        if page_size.is_some() {
+            within_bounds(&|| "the paged file-space strategy".into(), LibVer::V110)?;
+        }
+        // Versions 3 of the layout message and 2 of the superblock are what
+        // HDF5 1.8 reads; 1.10 added version 4 (with its chunk indexes) and
+        // version 3. A paged file needs the version-3 superblock whatever
+        // the low bound (libhdf5 raises it as far as the high bound allows).
+        let layout_version: u8 = if low < LibVer::V110 { 3 } else { 4 };
+        let superblock_version: u8 = if low < LibVer::V110 && page_size.is_none() {
+            2
+        } else {
+            3
+        };
+
         // The group tree, in layout order: groups depth-first from the root,
         // then every group's datasets in the same order.
         let tree = writer_tree::build(self.root, self.track_order)?;
@@ -1621,9 +1686,20 @@ impl FileWriter {
         let ds_attrs = all_ds.iter().flat_map(|d| &d.attrs);
         for a in group_attrs.chain(ds_attrs) {
             a.datatype.check_encodable()?;
+            within_bounds(
+                &|| format!("the datatype of attribute {:?}", a.name),
+                LibVer::for_datatype_version(a.datatype.max_encoded_version()),
+            )?;
         }
         for d in &all_ds {
             d.dt.check_encodable()?;
+            within_bounds(
+                &|| "a dataset's datatype".into(),
+                LibVer::for_datatype_version(d.dt.max_encoded_version()),
+            )?;
+            if d.virtual_sources.is_some() {
+                within_bounds(&|| "a virtual dataset".into(), LibVer::V110)?;
+            }
         }
 
         let is_vds: Vec<bool> = all_ds.iter().map(|d| d.virtual_sources.is_some()).collect();
@@ -1749,10 +1825,11 @@ impl FileWriter {
                     elem_size,
                     &d.chunk_options,
                 )?;
-                let result = build_chunked_data_from_precompressed(
+                let result = build_chunked_data_from_precompressed_libver(
                     &pre,
                     dummy_cursor,
                     d.maxshape.as_deref(),
+                    low,
                 )?;
                 dummy_cursor += result.data_bytes.len() as u64;
                 let oh = build_chunked_dataset_oh(
@@ -1785,6 +1862,7 @@ impl FileWriter {
                     },
                     &d.fill_message,
                     d.refcount,
+                    layout_version,
                 )?;
                 dummy_blobs.push(DataBlob {
                     data: vec![],
@@ -1804,6 +1882,7 @@ impl FileWriter {
                     },
                     &d.fill_message,
                     d.refcount,
+                    layout_version,
                 )?;
                 dummy_blobs.push(DataBlob {
                     data: vec![],
@@ -1904,13 +1983,14 @@ impl FileWriter {
                 let base_address = cursor2 as u64;
                 // Reuse precompressed chunks from Pass 1 — avoids re-compressing
                 // the same data a second time.
-                let result = build_chunked_data_from_precompressed(
+                let result = build_chunked_data_from_precompressed_libver(
                     dummy_blobs[i]
                         .precompressed
                         .as_ref()
                         .expect("chunked dataset missing precompressed cache"),
                     base_address,
                     d.maxshape.as_deref(),
+                    low,
                 )?;
                 cursor2 += result.data_bytes.len();
                 let oh = build_chunked_dataset_oh(
@@ -1944,6 +2024,7 @@ impl FileWriter {
                     },
                     &d.fill_message,
                     d.refcount,
+                    layout_version,
                 )?;
                 ds_blobs2.push(DataBlob {
                     data: vec![],
@@ -1973,6 +2054,7 @@ impl FileWriter {
                     },
                     &d.fill_message,
                     d.refcount,
+                    layout_version,
                 )?;
                 let mut data = vec![0u8; padding];
                 data.extend_from_slice(&d.raw);
@@ -1997,7 +2079,7 @@ impl FileWriter {
         let mut buf = Vec::with_capacity(cursor2);
 
         let sb = Superblock {
-            version: 3,
+            version: superblock_version,
             offset_size: OFFSET_SIZE,
             length_size: LENGTH_SIZE,
             base_address: 0,
@@ -2783,4 +2865,150 @@ mod tests {
         assert_eq!(sb.version, 3);
         assert_eq!(sb.page_size, None);
     }
+
+    fn layout_of(bytes: &[u8], name: &str) -> Vec<u8> {
+        let sb = Superblock::parse(bytes, 0).unwrap();
+        let addr = resolve_path_any(bytes, &sb, name).unwrap();
+        let hdr = ObjectHeader::parse(bytes, addr as usize, 8, 8).unwrap();
+        hdr.messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::DataLayout)
+            .unwrap()
+            .data
+            .clone()
+    }
+
+    #[test]
+    fn libver_v18_writes_the_1_8_format() {
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V18, LibVer::V18);
+        fw.create_dataset("contig").with_f64_data(&[1.0, 2.0]);
+        fw.create_dataset("compact").with_f64_data(&[3.0]).compact();
+        fw.create_dataset("grow")
+            .with_f64_data(&[1.0, 2.0, 3.0])
+            .with_maxshape(&[u64::MAX])
+            .with_chunks(&[2]);
+        fw.create_dataset("none")
+            .with_f64_data(&[])
+            .with_maxshape(&[u64::MAX])
+            .with_chunks(&[2]);
+        let bytes = fw.finish().unwrap();
+        assert_eq!(Superblock::parse(&bytes, 0).unwrap().version, 2);
+        assert_eq!(layout_of(&bytes, "contig")[..2], [3, 1]);
+        assert_eq!(layout_of(&bytes, "compact")[..2], [3, 0]);
+        let grow = layout_of(&bytes, "grow");
+        // Version 3, chunked, 2 dimensions (the element size is the last),
+        // B-tree address, chunk dims 2 and 8.
+        assert_eq!(grow[..3], [3, 2, 2]);
+        assert_eq!(grow[11..], [2, 0, 0, 0, 8, 0, 0, 0]);
+        let root = u64::from_le_bytes(grow[3..11].try_into().unwrap()) as usize;
+        assert_eq!(&bytes[root..root + 5], b"TREE\x01");
+        // No chunks, no tree.
+        assert_eq!(layout_of(&bytes, "none")[3..11], [0xff; 8]);
+        assert_eq!(read_dataset_f64(&bytes, "grow"), vec![1.0, 2.0, 3.0]);
+        assert_eq!(read_dataset_f64(&bytes, "contig"), vec![1.0, 2.0]);
+        assert_eq!(read_dataset_f64(&bytes, "compact"), vec![3.0]);
+    }
+
+    #[test]
+    fn default_libver_bounds_keep_the_1_10_format() {
+        let mut fw = FileWriter::new();
+        fw.create_dataset("contig").with_f64_data(&[1.0, 2.0]);
+        fw.create_dataset("grow")
+            .with_f64_data(&[1.0, 2.0, 3.0])
+            .with_maxshape(&[u64::MAX])
+            .with_chunks(&[2]);
+        let default = fw.finish().unwrap();
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V110, LibVer::Latest);
+        fw.create_dataset("contig").with_f64_data(&[1.0, 2.0]);
+        fw.create_dataset("grow")
+            .with_f64_data(&[1.0, 2.0, 3.0])
+            .with_maxshape(&[u64::MAX])
+            .with_chunks(&[2]);
+        assert_eq!(fw.finish().unwrap(), default);
+        assert_eq!(layout_of(&default, "contig")[0], 4);
+        assert_eq!(layout_of(&default, "grow")[..2], [4, 2]);
+    }
+
+    #[test]
+    fn libver_high_bound_refuses_newer_features() {
+        let bound = |r: Result<Vec<u8>, FormatError>, needs: LibVer| match r {
+            Err(FormatError::LibverBound { needs: n, high, .. }) => {
+                assert_eq!((n, high), (needs, LibVer::V18));
+            }
+            other => panic!("expected a bound error, got {other:?}"),
+        };
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V18, LibVer::V18);
+        fw.create_dataset("z")
+            .with_native_complex_f64_data(&[[1.0, 2.0]]);
+        bound(fw.finish(), LibVer::V200);
+
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V18, LibVer::V18);
+        fw.create_dataset("x").with_f64_data(&[1.0]).set_attr(
+            "z",
+            AttrValue::Raw {
+                datatype: crate::type_builders::make_native_complex_f64_type(),
+                shape: vec![],
+                data: vec![0; 16],
+            },
+        );
+        bound(fw.finish(), LibVer::V200);
+
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V18, LibVer::V18);
+        fw.create_dataset("r").with_compound_data(
+            Datatype::Reference {
+                size: 16,
+                ref_type: crate::datatype::ReferenceType::Object2,
+            },
+            vec![0; 16],
+            1,
+        );
+        bound(fw.finish(), LibVer::V112);
+
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V18, LibVer::V18);
+        fw.create_dataset("src").with_f64_data(&[1.0, 2.0]);
+        fw.create_dataset("vds")
+            .with_shape(&[2])
+            .with_f64_data(&[])
+            .with_virtual_sources(vec![VdsMapping {
+                source_file: ".".into(),
+                source_dataset: "src".into(),
+                source_selection: sel_all(),
+                virtual_selection: sel_hyper_1d(0, 2),
+            }]);
+        bound(fw.finish(), LibVer::V110);
+
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V18, LibVer::V18)
+            .with_page_size(4096);
+        bound(fw.finish(), LibVer::V110);
+
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V110, LibVer::V18);
+        bound(fw.finish(), LibVer::V110);
+    }
+
+    #[test]
+    fn libver_low_v18_high_latest_allows_newer_objects() {
+        // As libhdf5 does: the object that needs a newer format gets it,
+        // the rest of the file keeps the 1.8 format.
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V18, LibVer::Latest);
+        fw.create_dataset("z")
+            .with_native_complex_f64_data(&[[1.0, 2.0]]);
+        let bytes = fw.finish().unwrap();
+        assert_eq!(Superblock::parse(&bytes, 0).unwrap().version, 2);
+        let mut fw = FileWriter::new();
+        fw.libver_bounds(LibVer::V18, LibVer::Latest)
+            .with_page_size(4096);
+        fw.create_dataset("x").with_f64_data(&[1.0]);
+        let bytes = fw.finish().unwrap();
+        assert_eq!(Superblock::parse(&bytes, 0).unwrap().version, 3);
+        assert_eq!(layout_of(&bytes, "x")[0], 3);
+    }
 }
diff --git a/crates/clawhdf5-format/src/lib.rs b/crates/clawhdf5-format/src/lib.rs
index 9ce3ff9..32c3834 100644
--- a/crates/clawhdf5-format/src/lib.rs
+++ b/crates/clawhdf5-format/src/lib.rs
@@ -61,6 +61,7 @@ pub mod addr;
 pub mod attribute;
 pub mod attribute_info;
 pub mod btree_v1;
+mod btree_v1_write;
 pub mod btree_v2;
 mod btree_v2_write;
 mod bulk_alloc;
@@ -107,6 +108,7 @@ pub mod group_v1;
 pub mod group_v2;
 #[cfg(feature = "parallel")]
 pub mod lane_partition;
+pub mod libver;
 pub mod link_info;
 pub mod link_message;
 pub mod local_heap;
diff --git a/crates/clawhdf5-format/src/libver.rs b/crates/clawhdf5-format/src/libver.rs
new file mode 100644
index 0000000..42fe4fa
--- /dev/null
+++ b/crates/clawhdf5-format/src/libver.rs
@@ -0,0 +1,93 @@
+//! Library version bounds for writing: which HDF5 releases can read a file.
+//!
+//! libhdf5 picks the version of every object it writes from the file's
+//! *low* bound (`H5Pset_libver_bounds`; h5py's `libver=`): the oldest
+//! format version that holds the object, but never older than the one the
+//! low bound names. The *high* bound caps it: a feature that needs a newer
+//! format than the high bound is an error. [`LibVer`] names the same
+//! releases, and [`crate::file_writer::FileWriter::libver_bounds`] sets them.
+//!
+//! What the low bound changes in what clawhdf5 writes:
+//!
+//! | | low [`LibVer::V18`] | low [`LibVer::V110`] or later (the default) |
+//! |---|---|---|
+//! | superblock | version 2 | version 3 |
+//! | data layout message | version 3 | version 4 |
+//! | chunk index | version-1 B-tree (every chunked dataset) | single chunk, Fixed Array, Extensible Array or version-2 B-tree, as libhdf5 picks |
+//!
+//! Everything else (version-2 object headers, link and group-info messages,
+//! dense storage in fractal heaps with version-2 B-trees, filter pipeline
+//! version 2, fill value version 3, datatype versions up to 3) is the same
+//! and already readable by HDF5 1.8.
+//!
+//! What the high bound refuses: anything that needs 1.10 (virtual datasets,
+//! the paged file-space strategy) above [`LibVer::V18`], the 1.12 reference
+//! types (datatype version 4) above [`LibVer::V110`], and HDF5 2.0's native
+//! complex numbers (datatype version 5) above [`LibVer::V114`].
+//! `libver_bounds(LibVer::V18, LibVer::V18)` therefore writes a file HDF5
+//! 1.8 can read, or fails.
+
+use core::fmt;
+
+/// An HDF5 library release, as a bound on the file format versions a writer
+/// may use (libhdf5's `H5F_libver_t`). Ordered oldest first.
+///
+/// There is no `Earliest`: clawhdf5 cannot write the pre-1.8 format
+/// (symbol-table groups, version-1 object headers).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[non_exhaustive]
+pub enum LibVer {
+    /// HDF5 1.8 (`H5F_LIBVER_V18`, h5py `'v108'`).
+    V18,
+    /// HDF5 1.10 (`H5F_LIBVER_V110`, h5py `'v110'`).
+    V110,
+    /// HDF5 1.12 (`H5F_LIBVER_V112`, h5py `'v112'`).
+    V112,
+    /// HDF5 1.14 (`H5F_LIBVER_V114`, h5py `'v114'`).
+    V114,
+    /// HDF5 2.0 (`H5F_LIBVER_V200`).
+    V200,
+    /// The newest format this build of clawhdf5 writes
+    /// (`H5F_LIBVER_LATEST`, h5py `'latest'`).
+    Latest,
+}
+
+impl LibVer {
+    /// The release a datatype message of this version first appeared in:
+    /// versions 1-3 are readable by HDF5 1.8, 4 needs 1.12, 5 needs 2.0.
+    pub(crate) fn for_datatype_version(version: u8) -> Self {
+        match version {
+            0..=3 => LibVer::V18,
+            4 => LibVer::V112,
+            _ => LibVer::V200,
+        }
+    }
+}
+
+impl fmt::Display for LibVer {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            LibVer::V18 => "1.8",
+            LibVer::V110 => "1.10",
+            LibVer::V112 => "1.12",
+            LibVer::V114 => "1.14",
+            LibVer::V200 => "2.0",
+            LibVer::Latest => "latest",
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn ordered_oldest_first() {
+        assert!(LibVer::V18 < LibVer::V110);
+        assert!(LibVer::V114 < LibVer::V200);
+        assert!(LibVer::V200 < LibVer::Latest);
+        assert_eq!(LibVer::for_datatype_version(3), LibVer::V18);
+        assert_eq!(LibVer::for_datatype_version(4), LibVer::V112);
+        assert_eq!(LibVer::for_datatype_version(5), LibVer::V200);
+    }
+}
diff --git a/crates/clawhdf5-tools/tests/libver_v18.rs b/crates/clawhdf5-tools/tests/libver_v18.rs
new file mode 100644
index 0000000..0d27a39
--- /dev/null
+++ b/crates/clawhdf5-tools/tests/libver_v18.rs
@@ -0,0 +1,736 @@
+//! Files written with `FileBuilder::libver_bounds(LibVer::V18, LibVer::V18)`
+//! must be readable by HDF5 1.8. Every writer feature is written under that
+//! bound and read back by HDF5 1.8.23's h5dump (values dumped in binary and
+//! compared), by h5py (libhdf5 2.x), by h5dump 1.14, by clawhdf5 and by
+//! `h5rs check --data`; then `FileEditor` grows, appends to and annotates
+//! the file (splitting version-1 B-tree nodes) and every reader checks it
+//! again. The version-1 B-trees we write are compared node by node with
+//! the ones libhdf5 writes for the same data under h5py's
+//! `libver=('v108', 'latest')`.
+//!
+//! HDF5 1.8 is found through `CLAWHDF5_H5DUMP18` (the path to its h5dump)
+//! or at `~/.cache/hdf5-1.8.23/bin/h5dump`, where
+//! `scripts/build-hdf5-1.8.sh` builds it; without it the 1.8 checks are
+//! skipped (CI has no HDF5 1.8), even with `CLAWHDF5_REQUIRE_INTEROP=1`.
+//! h5py/numpy (`CLAWHDF5_PYTHON`) and h5dump are needed otherwise; they
+//! skip when missing unless `CLAWHDF5_REQUIRE_INTEROP=1`.
+
+use std::path::{Path, PathBuf};
+use std::process::{Command, Output};
+
+use clawhdf5::{AttrValue, File, FileBuilder, FileEditor, LibVer, Selection};
+use clawhdf5_format::datatype::{CharacterSet, Datatype, StringPadding};
+use clawhdf5_format::file_writer::{CompoundTypeBuilder, EnumTypeBuilder};
+use clawhdf5_format::type_builders::make_i32_type;
+
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
+
+fn interop_required() -> bool {
+    std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1")
+}
+
+fn available(cmd: &str, args: &[&str]) -> bool {
+    Command::new(cmd)
+        .args(args)
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false)
+}
+
+fn tools_ok() -> bool {
+    let ok =
+        available(&python(), &["-c", "import h5py, numpy"]) && available("h5dump", &["--version"]);
+    if !ok {
+        assert!(
+            !interop_required(),
+            "CLAWHDF5_REQUIRE_INTEROP=1 but h5py/numpy or h5dump is not available"
+        );
+        eprintln!("SKIP: h5py/numpy or h5dump not available");
+    }
+    ok
+}
+
+/// HDF5 1.8's h5dump, when there is one.
+fn h5dump18() -> Option<PathBuf> {
+    let p = match std::env::var_os("CLAWHDF5_H5DUMP18") {
+        Some(p) => PathBuf::from(p),
+        None => PathBuf::from(std::env::var_os("HOME")?).join(".cache/hdf5-1.8.23/bin/h5dump"),
+    };
+    let o = Command::new(&p).arg("--version").output().ok()?;
+    let v = String::from_utf8_lossy(&o.stdout).to_string();
+    if !v.contains("1.8.") {
+        eprintln!("SKIP 1.8 checks: {} is not HDF5 1.8 ({v})", p.display());
+        return None;
+    }
+    Some(p)
+}
+
+fn py(script: &str) -> String {
+    let o = Command::new(python())
+        .args(["-c", script])
+        .output()
+        .expect("run python");
+    assert!(
+        o.status.success(),
+        "python failed:\n{script}\nSTDOUT: {}\nSTDERR: {}",
+        String::from_utf8_lossy(&o.stdout),
+        String::from_utf8_lossy(&o.stderr)
+    );
+    String::from_utf8_lossy(&o.stdout).trim().to_string()
+}
+
+fn text(o: &Output) -> String {
+    format!(
+        "{}{}",
+        String::from_utf8_lossy(&o.stdout),
+        String::from_utf8_lossy(&o.stderr)
+    )
+}
+
+fn tmpdir() -> tempfile::TempDir {
+    tempfile::TempDir::new_in(env!("CARGO_TARGET_TMPDIR")).unwrap()
+}
+
+/// The hyperslab of `count` elements from `start`.
+fn block(start: &[u64], count: &[u64]) -> Selection {
+    Selection::Hyperslab {
+        start: start.to_vec(),
+        stride: vec![1; start.len()],
+        count: count.to_vec(),
+        block: vec![1; start.len()],
+    }
+}
+
+fn le<T: Copy, const N: usize>(v: &[T], f: impl Fn(T) -> [u8; N]) -> Vec<u8> {
+    v.iter().flat_map(|&x| f(x)).collect()
+}
+
+/// The datasets of the test file and the bytes each holds (little-endian,
+/// row-major, as h5py's `tobytes()` and h5dump's `-b LE` give them).
+struct Expect {
+    datasets: Vec<(String, Vec<u8>)>,
+}
+
+impl Expect {
+    fn set(&mut self, name: &str, bytes: Vec<u8>) {
+        match self.datasets.iter_mut().find(|(n, _)| n == name) {
+            Some(e) => e.1 = bytes,
+            None => self.datasets.push((name.to_string(), bytes)),
+        }
+    }
+}
+
+const MANY: usize = 100_000;
+
+/// Write every feature under the 1.8 bound.
+fn write_file(path: &Path) -> Expect {
+    let mut e = Expect {
+        datasets: Vec::new(),
+    };
+    let mut b = FileBuilder::new();
+    b.libver_bounds(LibVer::V18, LibVer::V18);
+
+    // Contiguous, with dense attributes (more than 8).
+    let v: Vec<f64> = (0..1000).map(|i| i as f64 * 0.5).collect();
+    let d = b.create_dataset("contig").with_f64_data(&v);
+    for i in 0..12 {
+        d.set_attr(&format!("a{i:02}"), AttrValue::I64(i));
+    }
+    e.set("/contig", le(&v, f64::to_le_bytes));
+    b.create_dataset("empty").with_f64_data(&[]);
+    e.set("/empty", vec![]);
+    b.create_dataset("scalar")
+        .with_f64_data(&[2.5])
+        .with_shape(&[]);
+    e.set("/scalar", 2.5f64.to_le_bytes().to_vec());
+    let v: Vec<i32> = (0..16).map(|i| i * 3 - 7).collect();
+    b.create_dataset("compact").with_i32_data(&v).compact();
+    e.set("/compact", le(&v, i32::to_le_bytes));
+    let v: Vec<f32> = (0..20).map(|i| i as f32 / 3.0).collect();
+    b.create_dataset("f32").with_f32_data(&v);
+    e.set("/f32", le(&v, f32::to_le_bytes));
+    let v: Vec<f32> = vec![0.5, -2.0, 1024.0, 0.0];
+    b.create_dataset("f16").with_f16_data(&v);
+    e.set(
+        "/f16",
+        le(&v, |x| {
+            clawhdf5_format::float16::f32_to_f16_bits(x).to_le_bytes()
+        }),
+    );
+
+    // Chunked with every built-in filter HDF5 1.8 has, and edge chunks.
+    let v: Vec<f32> = (0..60 * 70).map(|i| (i % 97) as f32 * 1.25).collect();
+    b.create_dataset("chunked")
+        .with_f32_data(&v)
+        .with_shape(&[60, 70])
+        .with_chunks(&[16, 16])
+        .with_deflate(6)
+        .with_shuffle()
+        .with_fletcher32();
+    e.set("/chunked", le(&v, f32::to_le_bytes));
+    // What the 1.10 indexes would be: Extensible Array, version-2 B-tree,
+    // Fixed Array, single chunk. All become version-1 B-trees.
+    let v: Vec<i32> = (0..25).collect();
+    b.create_dataset("resizable")
+        .with_i32_data(&v)
+        .with_maxshape(&[u64::MAX])
+        .with_chunks(&[4]);
+    e.set("/resizable", le(&v, i32::to_le_bytes));
+    let v: Vec<i64> = (0..30).map(|i| i * 1_000_000_007).collect();
+    b.create_dataset("resizable2")
+        .with_i64_data(&v)
+        .with_shape(&[5, 6])
+        .with_maxshape(&[u64::MAX, u64::MAX])
+        .with_chunks(&[2, 4]);
+    e.set("/resizable2", le(&v, i64::to_le_bytes));
+    let v: Vec<i32> = (0..10).map(|i| -i).collect();
+    b.create_dataset("fixedmax")
+        .with_i32_data(&v)
+        .with_maxshape(&[100])
+        .with_chunks(&[3])
+        .with_deflate(1);
+    e.set("/fixedmax", le(&v, i32::to_le_bytes));
+    let v: Vec<i32> = (0..8).map(|i| i * i).collect();
+    b.create_dataset("single")
+        .with_i32_data(&v)
+        .with_chunks(&[8]);
+    e.set("/single", le(&v, i32::to_le_bytes));
+    // Enough chunks for a three-level tree, and a 2-D two-level one.
+    let v: Vec<i32> = (0..MANY as i32).map(|i| i ^ 0x5a5a).collect();
+    b.create_dataset("many")
+        .with_i32_data(&v)
+        .with_maxshape(&[u64::MAX])
+        .with_chunks(&[1]);
+    e.set("/many", le(&v, i32::to_le_bytes));
+    let v: Vec<f64> = (0..100 * 100).map(|i| i as f64).collect();
+    b.create_dataset("grid")
+        .with_f64_data(&v)
+        .with_shape(&[100, 100])
+        .with_chunks(&[1, 1]);
+    e.set("/grid", le(&v, f64::to_le_bytes));
+    b.create_dataset("empty_chunked")
+        .with_f64_data(&[])
+        .with_maxshape(&[u64::MAX])
+        .with_chunks(&[10]);
+    e.set("/empty_chunked", vec![]);
+    let v: Vec<i32> = (0..12).collect();
+    b.create_dataset("filled")
+        .with_i32_data(&v)
+        .with_maxshape(&[u64::MAX])
+        .with_chunks(&[5])
+        .with_fill_value(&(-9i32).to_le_bytes());
+    e.set("/filled", le(&v, i32::to_le_bytes));
+
+    // Datatypes.
+    let raw = b"abcdehello\0\0\0\0\0".to_vec();
+    b.create_dataset("strings").with_compound_data(
+        Datatype::String {
+            size: 5,
+            padding: StringPadding::NullPad,
+            charset: CharacterSet::Ascii,
+        },
+        raw.clone(),
+        3,
+    );
+    e.set("/strings", raw);
+    let ct = CompoundTypeBuilder::new()
+        .i32_field("a")
+        .f64_field("b")
+        .build();
+    let mut raw = Vec::new();
+    for i in 0..5i32 {
+        raw.extend_from_slice(&i.to_le_bytes());
+        raw.extend_from_slice(&(f64::from(i) * 1.5).to_le_bytes());
+    }
+    b.create_dataset("compound")
+        .with_compound_data(ct, raw.clone(), 5);
+    e.set("/compound", raw);
+    let et = EnumTypeBuilder::i32_based()
+        .value("RED", 0)
+        .value("GREEN", 1)
+        .value("BLUE", 7)
+        .build();
+    let v = [0, 7, 1, 1, 0];
+    b.create_dataset("enum").with_enum_i32_data(et, &v);
+    e.set("/enum", le(&v, i32::to_le_bytes));
+    let v: Vec<i32> = (0..24).collect();
+    b.create_dataset("array").with_array_data(
+        make_i32_type(),
+        &[2, 3],
+        le(&v, i32::to_le_bytes),
+        4,
+    );
+    e.set("/array", le(&v, i32::to_le_bytes));
+    let v: Vec<i64> = vec![-1, 0, i64::MAX];
+    b.create_dataset("i64").with_i64_data(&v);
+    e.set("/i64", le(&v, i64::to_le_bytes));
+
+    // Groups: compact with links of every kind, dense (links and
+    // attributes), and tracking creation order.
+    let mut g = b.create_group("g_compact");
+    g.create_dataset("x").with_i32_data(&[1, 2, 3]);
+    e.set("/g_compact/x", le(&[1i32, 2, 3], i32::to_le_bytes));
+    g.add_soft_link("soft", "/contig");
+    g.add_external_link("ext", "other.h5", "/data");
+    g.set_attr("title", AttrValue::String("compact group".into()));
+    b.add_group(g.finish());
+    b.add_hard_link("hard", "/g_compact/x");
+    let mut g = b.create_group("g_dense");
+    for i in 0..20 {
+        let v = [i, i + 1];
+        g.create_dataset(&format!("d{i:02}")).with_i32_data(&v);
+        e.set(&format!("/g_dense/d{i:02}"), le(&v, i32::to_le_bytes));
+    }
+    for i in 0..12 {
+        g.set_attr(&format!("attr{i:02}"), AttrValue::F64(f64::from(i) / 4.0));
+    }
+    b.add_group(g.finish());
+    let mut g = b.create_group("g_order");
+    g.track_order(true);
+    for i in 0..10 {
+        let n = format!("z{}", 9 - i);
+        g.create_dataset(&n).with_i32_data(&[i]);
+        e.set(&format!("/g_order/{n}"), i.to_le_bytes().to_vec());
+    }
+    for i in 0..10 {
+        g.set_attr(&format!("b{}", 9 - i), AttrValue::I64(i));
+    }
+    b.add_group(g.finish());
+    b.create_dataset("deep/er/path").with_i32_data(&[42]);
+    e.set("/deep/er/path", 42i32.to_le_bytes().to_vec());
+
+    b.set_attr("version", AttrValue::F64(1.8));
+    b.set_attr("ints", AttrValue::I64Array(vec![1, -2, 3]));
+    b.set_attr("text", AttrValue::String("readable by 1.8".into()));
+    b.set_attr(
+        "texts",
+        AttrValue::StringArray(vec!["a".into(), "bb".into(), "ccc".into()]),
+    );
+    b.set_attr("big", AttrValue::U64(u64::MAX));
+    b.write(path).unwrap();
+    e
+}
+
+/// Structural facts of a file: superblock and layout message versions.
+fn check_versions(path: &Path) {
+    use clawhdf5_format::message_type::MessageType;
+    use clawhdf5_format::object_header::ObjectHeader;
+    let bytes = std::fs::read(path).unwrap();
+    assert_eq!(bytes[8], 2, "superblock version");
+    let f = File::open(path).unwrap();
+    let sb = f.superblock().clone();
+    for name in [
+        "contig",
+        "compact",
+        "chunked",
+        "resizable",
+        "resizable2",
+        "many",
+    ] {
+        let addr = clawhdf5_format::group_v2::resolve_path_any(&bytes, &sb, name).unwrap();
+        let oh = ObjectHeader::parse(&bytes, addr as usize, 8, 8).unwrap();
+        let layout = oh
+            .messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::DataLayout)
+            .unwrap();
+        assert_eq!(layout.data[0], 3, "layout version of {name}");
+    }
+}
+
+/// clawhdf5 reads every dataset's bytes back.
+fn check_ours(path: &Path, e: &Expect) {
+    let f = File::open(path).unwrap();
+    for (name, want) in &e.datasets {
+        let got = f
+            .dataset(name)
+            .unwrap()
+            .read_selection(&Selection::All)
+            .unwrap();
+        assert!(got == *want, "our read of {name} differs");
+    }
+    let attrs = f.dataset("contig").unwrap().attrs().unwrap();
+    assert!((12..=13).contains(&attrs.len()), "{attrs:?}");
+}
+
+/// h5py (libhdf5 2.x) reads every dataset's bytes back.
+fn check_h5py(path: &Path, e: &Expect, dir: &Path) {
+    let mut script = format!(
+        "import h5py, numpy as np\nf = h5py.File({p:?}, 'r')\n",
+        p = path.to_str().unwrap()
+    );
+    for (i, (name, want)) in e.datasets.iter().enumerate() {
+        let exp = dir.join(format!("expect{i}.bin"));
+        std::fs::write(&exp, want).unwrap();
+        script.push_str(&format!(
+            "a = np.ascontiguousarray(f[{name:?}][()]).tobytes()\n\
+             assert a == open({x:?}, 'rb').read(), {name:?}\n",
+            x = exp.to_str().unwrap()
+        ));
+    }
+    script.push_str(
+        "assert f.attrs['text'] in (b'readable by 1.8', 'readable by 1.8')\n\
+         assert list(f.attrs['ints']) == [1, -2, 3]\n\
+         assert len(f['g_dense'].attrs) == 12 and len(f['g_dense']) == 20\n\
+         assert list(f['g_order']) == ['z9', 'z8', 'z7', 'z6', 'z5', 'z4', 'z3', 'z2', 'z1', 'z0']\n\
+         assert f['g_compact/soft'].shape == (1000,)\n\
+         assert f['resizable'].maxshape == (None,)\n\
+         assert f['filled'].fillvalue == -9\n\
+         print('ok')\n",
+    );
+    assert_eq!(py(&script), "ok");
+}
+
+/// h5dump (1.14) and `h5rs check --data` accept the file.
+fn check_tools(path: &Path) {
+    let p = path.to_str().unwrap();
+    let o = Command::new(env!("CARGO_BIN_EXE_h5rs"))
+        .args(["check", "--data", "-q", p])
+        .output()
+        .unwrap();
+    assert!(o.status.success(), "h5rs check --data {p}:\n{}", text(&o));
+    let o = Command::new("h5dump")
+        .args(["-o", "/dev/null", p])
+        .output()
+        .unwrap();
+    assert!(
+        o.status.success() && o.stderr.is_empty(),
+        "h5dump {p}:\n{}",
+        text(&o)
+    );
+}
+
+/// HDF5 1.8's h5dump reads the whole file exactly as h5dump 1.14 does, and
+/// dumps each numeric dataset's values as the bytes we wrote.
+fn check_18(h5dump: &Path, path: &Path, e: &Expect, dir: &Path) {
+    let p = path.to_str().unwrap();
+    let o = Command::new(h5dump).arg(p).output().unwrap();
+    assert!(
+        o.status.success() && o.stderr.is_empty(),
+        "h5dump 1.8 {p}:\n{}",
+        text(&o)
+    );
+    let out18 = String::from_utf8_lossy(&o.stdout).to_string();
+    assert!(out18.contains("EXTERNAL_LINK \"ext\""), "{out18}");
+    let o = Command::new("h5dump").arg(p).output().unwrap();
+    assert!(o.status.success(), "h5dump {p}:\n{}", text(&o));
+    // HDF5 1.8 has no name for IEEE half floats.
+    let out = String::from_utf8_lossy(&o.stdout).replace(
+        "H5T_IEEE_F16LE",
+        "16-bit little-endian floating-point 16-bit precision",
+    );
+    if let Some((n, (a, b))) = out18
+        .lines()
+        .zip(out.lines())
+        .enumerate()
+        .find(|(_, (a, b))| a != b)
+    {
+        panic!("h5dump 1.8 and h5dump differ at line {}:\n{a}\n{b}", n + 1);
+    }
+    assert_eq!(out18.lines().count(), out.lines().count());
+    // `-b` writes nothing for compounds, enums, arrays, strings and half
+    // floats (HDF5 1.8
+    // has no native half float), for h5py's files too: those are covered
+    // by the text above.
+    let skip = ["/f16", "/strings", "/compound", "/enum", "/array"];
+    for (i, (name, want)) in e.datasets.iter().enumerate() {
+        if want.is_empty() || skip.contains(&name.as_str()) {
+            continue;
+        }
+        let bin = dir.join(format!("dump18_{i}.bin"));
+        let o = Command::new(h5dump)
+            .args(["-d", name, "-b", "LE", "-o"])
+            .arg(&bin)
+            .arg(p)
+            .output()
+            .unwrap();
+        assert!(o.status.success(), "h5dump 1.8 -d {name}:\n{}", text(&o));
+        let got = std::fs::read(&bin).unwrap();
+        assert!(got == *want, "HDF5 1.8 read of {name} differs");
+    }
+}
+
+fn check_all(path: &Path, e: &Expect, dir: &Path, h5dump: Option<&Path>) {
+    check_ours(path, e);
+    check_h5py(path, e, dir);
+    check_tools(path);
+    if let Some(h) = h5dump {
+        check_18(h, path, e, dir);
+    }
+}
+
+#[test]
+fn every_feature_reads_in_hdf5_1_8() {
+    if !tools_ok() {
+        return;
+    }
+    let h18 = h5dump18();
+    let dir = tmpdir();
+    let path = dir.path().join("v18.h5");
+    let mut e = write_file(&path);
+    check_versions(&path);
+    check_all(&path, &e, dir.path(), h18.as_deref());
+
+    // FileEditor: grow the unlimited datasets (splitting B-tree nodes),
+    // overwrite values in filtered and unfiltered chunks, set attributes
+    // (compact and dense).
+    let mut ed = FileEditor::open(&path).unwrap();
+    let mut res: Vec<i32> = (0..25).collect();
+    for round in 0..300usize {
+        let n = res.len() as u64;
+        let add = 1 + (round % 9) as u64;
+        ed.resize("resizable", &[n + add]).unwrap();
+        let vals: Vec<i32> = (0..add).map(|k| (n + k) as i32 * 7 - 3).collect();
+        ed.write_values("resizable", &block(&[n], &[add]), &vals)
+            .unwrap();
+        res.extend(&vals);
+    }
+    e.set("/resizable", le(&res, i32::to_le_bytes));
+    let mut many: Vec<i32> = (0..MANY as i32).map(|i| i ^ 0x5a5a).collect();
+    ed.resize("many", &[MANY as u64 + 500]).unwrap();
+    let vals: Vec<i32> = (0..500).collect();
+    ed.write_values("many", &block(&[MANY as u64], &[500]), &vals)
+        .unwrap();
+    many.extend(&vals);
+    many[12_345] = -1;
+    ed.write_values("many", &block(&[12_345], &[1]), &[-1i32])
+        .unwrap();
+    e.set("/many", le(&many, i32::to_le_bytes));
+    let mut chunked: Vec<f32> = (0..60 * 70).map(|i| (i % 97) as f32 * 1.25).collect();
+    let row: Vec<f32> = (0..70).map(|i| -(i as f32)).collect();
+    ed.write_values("chunked", &block(&[31, 0], &[1, 70]), &row)
+        .unwrap();
+    chunked[31 * 70..32 * 70].copy_from_slice(&row);
+    e.set("/chunked", le(&chunked, f32::to_le_bytes));
+    ed.resize("resizable2", &[9, 6]).unwrap();
+    let mut r2: Vec<i64> = (0..30).map(|i| i * 1_000_000_007).collect();
+    r2.extend(std::iter::repeat_n(0, 24));
+    e.set("/resizable2", le(&r2, i64::to_le_bytes));
+    ed.set_attr("contig", "added", &AttrValue::F64(3.25))
+        .unwrap();
+    ed.set_attr("g_dense", "attr03", &AttrValue::F64(-1.0))
+        .unwrap();
+    ed.set_attr("/", "text", &AttrValue::String("edited".into()))
+        .unwrap();
+    drop(ed);
+
+    check_ours(&path, &e);
+    let f = File::open(&path).unwrap();
+    assert!(matches!(
+        f.dataset("contig").unwrap().attr("added").unwrap(),
+        Some(AttrValue::F64(v)) if v == 3.25
+    ));
+    drop(f);
+    // Everything but the root's "text" attribute is as check_h5py expects.
+    py(&format!(
+        "import h5py\nf = h5py.File({p:?}, 'r')\n\
+         assert f.attrs['text'] in (b'edited', 'edited')\n\
+         assert f['contig'].attrs['added'] == 3.25\n\
+         assert f['g_dense'].attrs['attr03'] == -1.0\n",
+        p = path.to_str().unwrap()
+    ));
+    let o = Command::new(env!("CARGO_BIN_EXE_h5rs"))
+        .args(["check", "--data", "-q", path.to_str().unwrap()])
+        .output()
+        .unwrap();
+    assert!(o.status.success(), "h5rs check after edits:\n{}", text(&o));
+    if let Some(h) = h18.as_deref() {
+        check_18(h, &path, &e, dir.path());
+    }
+
+    // h5py (libhdf5 2.x) goes on appending to the 1.8-format file, and 1.8
+    // still reads it.
+    py(&format!(
+        "import h5py, numpy as np\n\
+         with h5py.File({p:?}, 'r+') as f:\n\
+         \x20   d = f['resizable']\n\
+         \x20   n = d.shape[0]\n\
+         \x20   d.resize((n + 100,))\n\
+         \x20   d[n:] = np.arange(100, dtype='<i4') + 5000\n",
+        p = path.to_str().unwrap()
+    ));
+    res.extend((0..100).map(|k| 5000 + k));
+    e.set("/resizable", le(&res, i32::to_le_bytes));
+    check_ours(&path, &e);
+    if let Some(h) = h18.as_deref() {
+        check_18(h, &path, &e, dir.path());
+    }
+}
+
+// ---- The trees against libhdf5's ----
+
+/// One node of a version-1 chunk B-tree: level, and per key its stored
+/// size and offsets; children below.
+#[derive(Debug, PartialEq)]
+struct TreeNode {
+    level: u8,
+    keys: Vec<(u32, Vec<u64>)>,
+    children: Vec<TreeNode>,
+}
+
+fn read_tree(bytes: &[u8], addr: u64, ndims: usize, sizes: bool) -> TreeNode {
+    let a = addr as usize;
+    assert_eq!(&bytes[a..a + 4], b"TREE");
+    let level = bytes[a + 5];
+    let n = u16::from_le_bytes([bytes[a + 6], bytes[a + 7]]) as usize;
+    let mut p = a + 24;
+    let mut keys = Vec::new();
+    let mut kids = Vec::new();
+    for i in 0..=n {
+        let size = u32::from_le_bytes(bytes[p..p + 4].try_into().unwrap());
+        let offs = (0..ndims)
+            .map(|d| u64::from_le_bytes(bytes[p + 8 + 8 * d..p + 16 + 8 * d].try_into().unwrap()))
+            .collect();
+        keys.push((if sizes { size } else { 0 }, offs));
+        p += 8 + 8 * ndims;
+        if i < n {
+            kids.push(u64::from_le_bytes(bytes[p..p + 8].try_into().unwrap()));
+            p += 8;
+        }
+    }
+    let children = if level > 0 {
+        kids.iter()
+            .map(|&c| read_tree(bytes, c, ndims, sizes))
+            .collect()
+    } else {
+        Vec::new()
+    };
+    TreeNode {
+        level,
+        keys,
+        children,
+    }
+}
+
+/// Where two trees first differ (libhdf5's first).
+fn first_difference(a: &TreeNode, b: &TreeNode, at: &str) -> Option<String> {
+    if a.level != b.level || a.keys.len() != b.keys.len() {
+        return Some(format!(
+            "{at}: level {} with {} keys vs level {} with {} keys",
+            a.level,
+            a.keys.len(),
+            b.level,
+            b.keys.len()
+        ));
+    }
+    if let Some(i) = (0..a.keys.len()).find(|&i| a.keys[i] != b.keys[i]) {
+        return Some(format!("{at} key {i}: {:?} vs {:?}", a.keys[i], b.keys[i]));
+    }
+    a.children
+        .iter()
+        .zip(&b.children)
+        .enumerate()
+        .find_map(|(i, (x, y))| first_difference(x, y, &format!("{at}/{i}")))
+}
+
+/// The chunk B-tree of dataset `name`: (tree, ndims) from its layout.
+fn tree_of(path: &Path, name: &str, sizes: bool) -> TreeNode {
+    use clawhdf5_format::message_type::MessageType;
+    use clawhdf5_format::object_header::ObjectHeader;
+    let bytes = std::fs::read(path).unwrap();
+    let f = File::open(path).unwrap();
+    let sb = f.superblock().clone();
+    let addr = clawhdf5_format::group_v2::resolve_path_any(&bytes, &sb, name).unwrap();
+    let oh = ObjectHeader::parse(&bytes, addr as usize, 8, 8).unwrap();
+    let l = &oh
+        .messages
+        .iter()
+        .find(|m| m.msg_type == MessageType::DataLayout)
+        .unwrap()
+        .data;
+    assert_eq!((l[0], l[1]), (3, 2), "{name}: layout v3, chunked");
+    let ndims = l[2] as usize;
+    let root = u64::from_le_bytes(l[3..11].try_into().unwrap());
+    read_tree(&bytes, root, ndims, sizes)
+}
+
+/// Our version-1 chunk B-trees are libhdf5's, node for node (levels, child
+/// counts, every key's offsets, and its chunk size where the chunks are
+/// the same bytes), for 1-D, 2-D and 3-D datasets with two- and three-level
+/// trees, filtered or not.
+///
+/// libhdf5 inserts each chunk into the tree when it leaves its chunk cache.
+/// A whole-dataset write with no cache (`rdcc_nbytes=0`, or chunks larger
+/// than the cache) inserts them in row-major order, as we build the tree;
+/// with the default cache small chunks of a 1-D dataset still arrive in
+/// order, but those of a multi-dimensional one arrive in the order the
+/// cache's hash evicts them, which gives the same keys in differently
+/// filled nodes. Both trees index the same chunks; we do not model the
+/// cache.
+#[test]
+fn chunk_btrees_match_libhdf5() {
+    if !tools_ok() {
+        return;
+    }
+    let dir = tmpdir();
+    let theirs = dir.path().join("libhdf5.h5");
+    py(&format!(
+        "import h5py, numpy as np\n\
+         with h5py.File({p:?}, 'w', libver=('v108', 'latest'), rdcc_nbytes=0) as f:\n\
+         \x20   f.create_dataset('d1000', data=np.arange(10000.0), chunks=(10,))\n\
+         \x20   f.create_dataset('d999', data=np.arange(9990.0), chunks=(10,))\n\
+         \x20   f.create_dataset('big', data=np.arange(100000, dtype='<i4'), chunks=(1,), maxshape=(None,))\n\
+         \x20   f.create_dataset('grid', data=np.arange(10000.0).reshape(100, 100), chunks=(1, 1))\n\
+         \x20   f.create_dataset('cube', data=np.arange(27000, dtype='<i2').reshape(30, 30, 30), chunks=(2, 3, 5))\n\
+         \x20   f.create_dataset('gz', data=np.arange(20000, dtype='<i8') % 13, chunks=(7,), compression='gzip')\n",
+        p = theirs.to_str().unwrap()
+    ));
+    let ours = dir.path().join("ours.h5");
+    let mut b = FileBuilder::new();
+    b.libver_bounds(LibVer::V18, LibVer::V18);
+    let v: Vec<f64> = (0..10000).map(f64::from).collect();
+    b.create_dataset("d1000")
+        .with_f64_data(&v)
+        .with_chunks(&[10]);
+    b.create_dataset("d999")
+        .with_f64_data(&v[..9990])
+        .with_chunks(&[10]);
+    let v: Vec<i32> = (0..100_000).collect();
+    b.create_dataset("big")
+        .with_i32_data(&v)
+        .with_chunks(&[1])
+        .with_maxshape(&[u64::MAX]);
+    let v: Vec<f64> = (0..10000).map(f64::from).collect();
+    b.create_dataset("grid")
+        .with_f64_data(&v)
+        .with_shape(&[100, 100])
+        .with_chunks(&[1, 1]);
+    let raw: Vec<u8> = (0..27000i16).flat_map(|x| x.to_le_bytes()).collect();
+    b.create_dataset("cube")
+        .with_compound_data(
+            clawhdf5_format::datatype::Datatype::FixedPoint {
+                size: 2,
+                byte_order: clawhdf5_format::datatype::DatatypeByteOrder::LittleEndian,
+                signed: true,
+                bit_offset: 0,
+                bit_precision: 16,
+            },
+            raw,
+            27000,
+        )
+        .with_shape(&[30, 30, 30])
+        .with_chunks(&[2, 3, 5]);
+    let v: Vec<i64> = (0..20000).map(|i| i % 13).collect();
+    b.create_dataset("gz")
+        .with_i64_data(&v)
+        .with_chunks(&[7])
+        .with_deflate(4);
+    b.write(&ours).unwrap();
+    for (name, sizes) in [
+        ("d1000", true),
+        ("d999", true),
+        ("big", true),
+        ("grid", true),
+        ("cube", true),
+        // Compressed sizes differ between zlib-rs and zlib.
+        ("gz", false),
+    ] {
+        let a = tree_of(&theirs, name, sizes);
+        let b = tree_of(&ours, name, sizes);
+        if let Some(d) = first_difference(&a, &b, "root") {
+            panic!("{name}: our chunk B-tree differs from libhdf5's: {d}");
+        }
+    }
+}
diff --git a/crates/clawhdf5/src/lib.rs b/crates/clawhdf5/src/lib.rs
index 24bd435..9678c29 100644
--- a/crates/clawhdf5/src/lib.rs
+++ b/crates/clawhdf5/src/lib.rs
@@ -68,6 +68,7 @@ pub use writer::{DatasetSpec, create_datasets_parallel};
 // Re-export useful types from clawhdf5-format for advanced users
 pub use clawhdf5_format::data_layout::VdsMapping;
 pub use clawhdf5_format::dict_encoding::{DictEncoded, DictionaryEncoder};
+pub use clawhdf5_format::libver::LibVer;
 pub use clawhdf5_format::property_list::{
     DatasetCreateProps, FileAccessProps, FileCreateProps, lib_version,
 };
diff --git a/crates/clawhdf5/src/writer.rs b/crates/clawhdf5/src/writer.rs
index ea48bc9..9fcf6fc 100644
--- a/crates/clawhdf5/src/writer.rs
+++ b/crates/clawhdf5/src/writer.rs
@@ -1,6 +1,7 @@
 //! Writing API: FileBuilder and GroupBuilder for creating HDF5 files.
 
 use clawhdf5_format::file_writer::FileWriter as FormatWriter;
+use clawhdf5_format::libver::LibVer;
 use clawhdf5_format::type_builders::{
     AttrValue, DatasetBuilder as FormatDatasetBuilder, FinishedGroup,
     GroupBuilder as FormatGroupBuilder,
@@ -98,6 +99,34 @@ impl FileBuilder {
         self
     }
 
+    /// Set the library version bounds, as h5py's `libver=(low, high)`: the
+    /// oldest HDF5 release whose file format is used, and the newest whose
+    /// features are allowed. The default is `(LibVer::V110,
+    /// LibVer::Latest)`, the HDF5 1.10 format clawhdf5 has always written.
+    ///
+    /// `libver_bounds(LibVer::V18, LibVer::V18)` writes a file HDF5 1.8 can
+    /// read (version-1 B-tree chunk indexes, version-2 superblock, the low
+    /// bound libhdf5 2.0 uses by default), or fails with
+    /// [`Error::Format`] (`FormatError::LibverBound`) for anything 1.8
+    /// cannot read: virtual datasets, the 1.12 reference types, native
+    /// complex numbers. See `clawhdf5_format::libver` for the details.
+    ///
+    /// ```
+    /// use clawhdf5::{FileBuilder, LibVer};
+    ///
+    /// let mut b = FileBuilder::new();
+    /// b.libver_bounds(LibVer::V18, LibVer::V18);
+    /// b.create_dataset("x")
+    ///     .with_f64_data(&[1.0, 2.0, 3.0])
+    ///     .with_maxshape(&[u64::MAX]);
+    /// let bytes = b.finish().unwrap();
+    /// assert_eq!(bytes[8], 2); // superblock version 2
+    /// ```
+    pub fn libver_bounds(&mut self, low: LibVer, high: LibVer) -> &mut Self {
+        self.writer.libver_bounds(low, high);
+        self
+    }
+
     /// Set an attribute on the root group.
     pub fn set_attr(&mut self, name: &str, value: AttrValue) {
         self.writer.set_root_attr(name, value);
diff --git a/scripts/build-hdf5-1.8.sh b/scripts/build-hdf5-1.8.sh
new file mode 100644
index 0000000..cc75fc0
--- /dev/null
+++ b/scripts/build-hdf5-1.8.sh
@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+# Build libhdf5 1.8.23 (the last 1.8 release) with its command-line tools, as
+# the oracle for files written with `LibVer::V18` (the `libver_v18` test in
+# clawhdf5-tools finds h5dump through CLAWHDF5_H5DUMP18 or this default prefix).
+#
+#   bash scripts/build-hdf5-1.8.sh [PREFIX]
+#
+# PREFIX defaults to ~/.cache/hdf5-1.8.23; sources go to PREFIX-src and the
+# build tree to PREFIX-build. Reuses an existing install. Needs git, cmake, a C
+# compiler and zlib headers.
+set -euo pipefail
+PREFIX="${1:-$HOME/.cache/hdf5-1.8.23}"
+SRC="$PREFIX-src"
+BUILD="$PREFIX-build"
+if [ -x "$PREFIX/bin/h5dump" ]; then
+    echo "reusing $PREFIX/bin/h5dump"
+    "$PREFIX/bin/h5dump" --version
+    exit 0
+fi
+if [ ! -d "$SRC" ]; then
+    git clone --depth 1 --branch hdf5-1_8_23 https://github.com/HDFGroup/hdf5.git "$SRC"
+fi
+# 1.8 predates current compilers (GCC 14 turns its pointer-type mismatches in
+# the tools into errors): pin gnu99 and demote those errors to warnings.
+CFLAGS18="-w -std=gnu99 -Wno-error=incompatible-pointer-types"
+CFLAGS18="$CFLAGS18 -Wno-error=implicit-function-declaration -Wno-error=int-conversion"
+cmake -S "$SRC" -B "$BUILD" \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_INSTALL_PREFIX="$PREFIX" \
+    -DCMAKE_C_FLAGS="$CFLAGS18" \
+    -DBUILD_SHARED_LIBS=ON \
+    -DBUILD_TESTING=OFF \
+    -DHDF5_BUILD_TOOLS=ON \
+    -DHDF5_BUILD_EXAMPLES=OFF \
+    -DHDF5_BUILD_CPP_LIB=OFF \
+    -DHDF5_BUILD_FORTRAN=OFF \
+    -DHDF5_BUILD_HL_LIB=OFF \
+    -DHDF5_BUILD_JAVA=OFF \
+    -DHDF5_ENABLE_Z_LIB_SUPPORT=ON \
+    -DHDF5_ENABLE_SZIP_SUPPORT=OFF
+cmake --build "$BUILD" -j "${JOBS:-6}"
+cmake --install "$BUILD"
+"$PREFIX/bin/h5dump" --version