diff --git a/DevDocMD/implementation/partition_layer_cache.md b/DevDocMD/implementation/partition_layer_cache.md
new file mode 100644
index 00000000..f3208fcd
--- /dev/null
+++ b/DevDocMD/implementation/partition_layer_cache.md
@@ -0,0 +1,129 @@
+# Partition and layer caching (discussion)
+
+Status: problem confirmed, design direction agreed (2026-08-20). No
+implementation started. Ownership split (below) still open.
+
+## The problem
+
+Reading a layer's data (MPHF + matrix) is not free: `MphfLayer::open` mmaps
+`mphf.bin` plus (`evidence.bin`/`fingerprint.bin` + `unitigs.bin`), and the
+matrix side mmaps `matrix.pbmx`/`matrix.pcmx` (or one file per genome column
+if not yet packed). Any code path that reopens a layer per lookup instead of
+once per run pays this cost repeatedly.
+
+`obikphylo::siblings::cache::PartitionCache` was built to avoid exactly this
+for `build_sibling_annex`/`sibling_annex_stats`: those commands probe many
+partitions, once per source layer, over the whole run. Profiling a real run
+showed wall-clock time dominated by repeated `open()`/mmap syscalls, not
+computation — parallelising the naive per-lookup opens spread the cost
+across cores without reducing it. `PartitionCache::build` opens every
+partition's every layer once, up front, in parallel, and keeps the handles
+alive for the run.
+
+## Three independent implementations of the same bundle
+
+Searching the codebase for "who bundles MPHF + matrix, with per-layer format
+auto-detection" turns up three unrelated implementations:
+
+| | lives in | scope | cached? |
+|---|---|---|---|
+| `Layer<D>` | `obilayeredmap` | one layer, `D` fixed at compile time | held alive by whoever owns the `Layer`, no policy of its own |
+| `Mat` | `obikphylo::siblings::cache` | one layer, format resolved per instance from an enum of 3 `Layer<D>` variants | yes, via `PartitionCache` |
+| `QueryLayer` | `obikpartitionner::query_layer` | one layer, `(MphfLayer, PersistentBitMatrix\|PersistentCompactIntMatrix)` pair, bypasses `Layer<D>` entirely | **no** — opened fresh inside `query_partition_with` on every call |
+
+`query_partition_with` is `obikmer query`'s normal query path — the one
+most exposed to repeated cross-partition lookups — and it is the one with
+no cache at all. `obikphylo` built a cache first only because sibling-annex
+construction hits the cost hardest, not because the need is sibling-specific.
+
+## The gap in `obilayeredmap`'s existing cache
+
+`obilayeredmap::LayeredMap<D>` already caches correctly at the granularity
+of one partition: `open(root)` opens every layer once, keeps `Vec<Layer<D>>`
+alive for the `LayeredMap`'s lifetime. But it is monomorphic — every layer
+in the `Vec` must share the same concrete `D`. In practice this is false:
+layers in the same partition are packed independently over time (`pack
+--sparse` converts one layer's presence matrix at a time), so a partition
+can genuinely mix `PersistentBitMatrix`-backed and
+`PersistentSparseBitMatrix`-backed presence layers. `LayeredMap<D>` cannot
+represent that mix — `Mat`'s 3-way enum exists specifically to work around
+this gap, one crate away from where the gap actually is.
+
+## Resource cost: mmap does not hold a file descriptor
+
+Before deciding how many layers/partitions a cache may hold open
+simultaneously, the binding constraint needs to be identified correctly.
+
+Confirmed against upstream documentation, not inferred from behaviour:
+
+> "After the mmap() call has returned, the file descriptor, fd, can be
+> closed immediately without invalidating the mapping."
+> — [mmap(2), man7.org](https://man7.org/linux/man-pages/man2/mmap.2.html)
+
+> "The close(2) function does not unmap pages"
+> — [mmap(2), Apple Developer](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/mmap.2.html)
+
+> "A file backed Mmap ... will remain valid even after the File is dropped.
+> ... the Mmap handle is completely independent of the File used to create
+> it."
+> — [memmap2::Mmap, docs.rs](https://docs.rs/memmap2/latest/memmap2/struct.Mmap.html)
+
+Every read-only mmap in this codebase already follows this: `Mmap::map(&File::open(path)?)?`
+— the `File` is a temporary, dropped (fd closed) immediately after the
+mapping is established; every persistent struct (`PersistentBitVec`,
+`PersistentCompactIntVec`, `PackedBitMatrix`, `Evidence`, `FingerprintVec`,
+...) stores only the `Mmap`, never the `File`. So a cache built on these
+types does **not** consume the process's open-file-descriptor budget
+(`ulimit -n`, notoriously low by default on macOS) proportionally to how
+many mmapped files it holds.
+
+It does consume a different resource — the process's virtual-memory mapping
+table (one entry per active `mmap()` region). Linux exposes this as
+`vm.max_map_count` (default 65530). No documented macOS equivalent (fixed
+numeric ceiling) was found; the constraint there appears to be virtual
+address space rather than an explicit mapping counter, but this is not
+sourced and should not be assumed. This is the resource actually worth
+measuring before deciding on cache size, not fd count — and it is why
+*packing* (`matrix.pbmx`/`matrix.pcmx`, one mmap for all columns) matters
+independently of any caching decision: an unpacked `Columnar` matrix opens
+one mmap **per genome column**, multiplying the mapping count a cache would
+have to hold by `n_genomes`.
+
+## Layering: who owns what
+
+Established in discussion, not yet coded:
+
+- `obilayeredmap` operates within a single partition's index root; it has
+  no notion of "partition" (confirmed independently while fixing
+  `open_data`/`layer_dir` — see git history around 2026-08-20). One
+  `LayeredMap<D>` (or its heterogeneous-`D` successor) = one partition.
+- Nobody currently owns "the collection of partitions" as reusable state.
+  `obikpartitionner::KmerPartition` is the closest candidate — it already
+  owns `n_partitions()`/`part_dir(i)` — but today it is a pure
+  config/path resolver, not a state holder: `dereplicate`, `count_kmer`,
+  `obikindex`'s `distance.rs`/`stats.rs`, and
+  `obikphylo::PartitionCache::build` each independently write their own
+  `(0..n_partitions).into_par_iter().map(...)` loop and open what they need
+  from scratch.
+
+## Direction agreed, not yet implemented
+
+1. A heterogeneous-format layer type in `obilayeredmap` (name TBD),
+   generalising `Mat`'s 3-way enum + auto-detection — `obilayeredmap`
+   already implements `LayerData` for all three concrete matrix types
+   involved, so it already has the knowledge `Mat` needed and had to
+   duplicate.
+2. A cache spanning multiple partitions, keeping (2)'s layer type open per
+   partition per layer, for the whole lifetime of a long-running command —
+   generalising `PartitionCache` minus its sibling-specific parts
+   (`fast_mode`, `find_presence_batch`) — living in `obikpartitionner`
+   (owner of the partition dimension) and built on top of (1).
+3. `obikpartitionner::query_partition_with` — the normal query path,
+   currently uncached — becomes a consumer of (2), not just
+   `obikphylo::PartitionCache`.
+
+Open before implementing: exact API shape of (1) and (2), whether `Mat` is
+deleted outright or kept as a thin sibling-specific wrapper, and whether (2)
+needs an eviction policy or can simply hold every partition open for the
+process lifetime (revisit once the VM-mapping-count question above has a
+real number behind it for this codebase's scale).
diff --git a/mkdocs.yml b/mkdocs.yml
index b1660442..4fec9411 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -56,6 +56,7 @@ nav:
       - Select command: implementation/select.md
       - obitaxonomy crate: implementation/obitaxonomy.md
       - "Benchmark: query-path testing": implementation/benchmark_query_testing.md
+      - "Partition and layer caching (discussion)": implementation/partition_layer_cache.md
   - Architecture:
       - Sequences: architecture/sequences/invariant.md
       - Kmer index: architecture/index_architecture.md
diff --git a/src/obikindex/examples/compare_sparse.rs b/src/obikindex/examples/compare_sparse.rs
index 72534989..51376fad 100644
--- a/src/obikindex/examples/compare_sparse.rs
+++ b/src/obikindex/examples/compare_sparse.rs
@@ -5,7 +5,6 @@
 
 use obicompactvec::{PersistentBitMatrix, PersistentSparseBitMatrix};
 use obikindex::KmerIndex;
-use obilayeredmap::layer_dir;
 
 fn main() -> anyhow::Result<()> {
     let sparse_root = std::env::args().nth(1).expect("usage: compare_sparse <sparse_root> <dense_root>");
@@ -26,18 +25,16 @@ fn main() -> anyhow::Result<()> {
     let mut first_mismatch = None;
 
     for part in 0..n_parts {
-        let part_dir_sparse = sparse.partition().part_dir(part);
-        let part_dir_dense = dense.partition().part_dir(part);
+        let index_dir_sparse = sparse.partition().index_dir(part);
+        let index_dir_dense = dense.partition().index_dir(part);
 
-        if !part_dir_sparse.join("index").exists() || !part_dir_dense.join("index").exists() {
+        if !index_dir_sparse.exists() || !index_dir_dense.exists() {
             continue;
         }
 
         for layer in 0..n_layers {
-            let index_dir_sparse = part_dir_sparse.join("index");
-            let index_dir_dense = part_dir_dense.join("index");
-            let layer_dir_sparse = layer_dir(&index_dir_sparse, layer);
-            let layer_dir_dense = layer_dir(&index_dir_dense, layer);
+            let layer_dir_sparse = sparse.partition().layer_dir(part, layer);
+            let layer_dir_dense = dense.partition().layer_dir(part, layer);
 
             if !layer_dir_sparse.exists() || !layer_dir_dense.exists() {
                 continue;
diff --git a/src/obikindex/src/index.rs b/src/obikindex/src/index.rs
index cadc7440..d178113f 100644
--- a/src/obikindex/src/index.rs
+++ b/src/obikindex/src/index.rs
@@ -149,7 +149,7 @@ impl KmerIndex {
     /// is enough, no need to scan every partition.
     pub fn n_layers_per_partition(&self) -> OKIResult<usize> {
         use obilayeredmap::meta::PartitionMeta;
-        let index_dir = self.partition.part_dir(0).join("index");
+        let index_dir = self.partition.index_dir(0);
         let meta = PartitionMeta::load(&index_dir)
             .map_err(|e| OKIError::Io(std::io::Error::new(std::io::ErrorKind::Other, e.to_string())))?;
         Ok(meta.n_layers)
@@ -264,8 +264,7 @@ impl KmerIndex {
 
     /// Path to the unitigs file for partition `part`, layer `layer`.
     pub fn layer_unitigs_path(&self, part: usize, layer: usize) -> PathBuf {
-        let index_dir = self.partition.part_dir(part).join("index");
-        obilayeredmap::layer_dir(&index_dir, layer).join("unitigs.bin")
+        self.partition.layer_dir(part, layer).join("unitigs.bin")
     }
 
     /// Pack all partition matrices into single-file format (presence → .pbmx, counts → .pcmx).
@@ -288,12 +287,12 @@ impl KmerIndex {
         crate::numa::PartitionRunner::new().run(
             &order,
             |i| -> OKIResult<()> {
-                let index_dir = self.partition.part_dir(i).join("index");
+                let index_dir = self.partition.index_dir(i);
                 if !index_dir.exists() { return Ok(()); }
                 let meta = PartitionMeta::load(&index_dir)
                     .map_err(|e| OKIError::Io(std::io::Error::new(std::io::ErrorKind::Other, e.to_string())))?;
                 for l in 0..meta.n_layers {
-                    let layer_dir = obilayeredmap::layer_dir(&index_dir, l);
+                    let layer_dir = self.partition.layer_dir(i, l);
                     let presence_dir = layer_dir.join("presence");
                     let counts_dir   = layer_dir.join("counts");
                     if presence_dir.exists() {
@@ -326,14 +325,14 @@ impl KmerIndex {
         let errors: Vec<_> = (0..n)
             .into_par_iter()
             .filter_map(|i| {
-                let index_dir = self.partition.part_dir(i).join("index");
+                let index_dir = self.partition.index_dir(i);
                 if !index_dir.exists() { return None; }
                 let meta = match PartitionMeta::load(&index_dir) {
                     Ok(m) => m,
                     Err(e) => return Some(OKIError::Io(std::io::Error::new(std::io::ErrorKind::Other, e.to_string()))),
                 };
                 for l in 0..meta.n_layers {
-                    let layer_dir = obilayeredmap::layer_dir(&index_dir, l);
+                    let layer_dir = self.partition.layer_dir(i, l);
                     let meta_path = layer_dir.join(LayerMeta::FILENAME);
                     if meta_path.exists() { continue; }
                     let unitigs_path = layer_dir.join("unitigs.bin");
diff --git a/src/obikindex/src/reindex.rs b/src/obikindex/src/reindex.rs
index 7dbab9fd..05a919e1 100644
--- a/src/obikindex/src/reindex.rs
+++ b/src/obikindex/src/reindex.rs
@@ -48,7 +48,7 @@ impl KmerIndex {
         let runner = crate::numa::PartitionRunner::new();
         runner.run(
             &order,
-            |i| reindex_partition(&self.partition.part_dir(i).join("index"), &target, block_bits)
+            |i| reindex_partition(&self.partition.index_dir(i), &target, block_bits)
                 .map_err(|e| OKIError::InvalidInput(format!("partition {i}: {e}"))),
             |_, _, _| { pb.inc(1); },
         )?;
diff --git a/src/obikindex/src/stats.rs b/src/obikindex/src/stats.rs
index 4db896a9..eef2a252 100644
--- a/src/obikindex/src/stats.rs
+++ b/src/obikindex/src/stats.rs
@@ -3,7 +3,6 @@ use std::path::Path;
 
 use obicompactvec::{LayerMeta, PersistentBitMatrix, PersistentCompactIntMatrix};
 use obicompactvec::traits::ColumnWeights;
-use obilayeredmap::layer_dir;
 use obilayeredmap::meta::PartitionMeta;
 use rayon::prelude::*;
 
@@ -91,7 +90,7 @@ impl KmerIndex {
         let (n_kmers, mphf_b, evidence_b, matrix_b) = (0..n)
             .into_par_iter()
             .map(|i| {
-                let index_dir = self.partition.part_dir(i).join("index");
+                let index_dir = self.partition.index_dir(i);
                 if !index_dir.exists() { return (0usize, 0u64, 0u64, 0u64); }
 
                 let n_layers = PartitionMeta::load(&index_dir)
@@ -99,7 +98,7 @@ impl KmerIndex {
                     .unwrap_or(0);
 
                 (0..n_layers).fold((0usize, 0u64, 0u64, 0u64), |acc, l| {
-                    let lb = layer_bytes(&layer_dir(&index_dir, l));
+                    let lb = layer_bytes(&self.partition.layer_dir(i, l));
                     (acc.0 + lb.n_kmers, acc.1 + lb.mphf, acc.2 + lb.evidence, acc.3 + lb.matrix)
                 })
             })
@@ -142,7 +141,7 @@ impl KmerIndex {
                 let mut counts  = vec![0u64; n_genomes];
                 let mut n_kmers = 0usize;
 
-                let index_dir = self.partition.part_dir(i).join("index");
+                let index_dir = self.partition.index_dir(i);
                 if !index_dir.exists() { return (0, counts); }
 
                 let n_layers = PartitionMeta::load(&index_dir)
@@ -150,7 +149,7 @@ impl KmerIndex {
                     .unwrap_or(0);
 
                 for l in 0..n_layers {
-                    let this_layer_dir = obilayeredmap::layer_dir(&index_dir, l);
+                    let this_layer_dir = self.partition.layer_dir(i, l);
                     if !this_layer_dir.exists() { continue; }
 
                     n_kmers += LayerMeta::load(&this_layer_dir).map(|m| m.n).unwrap_or(0);
diff --git a/src/obikpartitionner/src/distance.rs b/src/obikpartitionner/src/distance.rs
index 3cc33aeb..837d2cdc 100644
--- a/src/obikpartitionner/src/distance.rs
+++ b/src/obikpartitionner/src/distance.rs
@@ -1,24 +1,22 @@
 use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
-use obilayeredmap::{layer_dir, open_data, LayeredStore};
+use obilayeredmap::{open_data, LayeredStore};
 use obiskio::SKResult;
 
 use crate::common::{load_meta, olm_to_sk};
 use crate::partition::KmerPartition;
 
-const INDEX_SUBDIR: &str = "index";
-
 impl KmerPartition {
     /// Open all count matrices for partition `part`, one per layer.
     /// Layers without a `counts/` directory are skipped.
     pub fn count_store(&self, part: usize) -> SKResult<LayeredStore<PersistentCompactIntMatrix>> {
-        let index_dir = self.part_dir(part).join(INDEX_SUBDIR);
+        let index_dir = self.index_dir(part);
         if !index_dir.exists() {
             return Ok(LayeredStore::new(vec![]));
         }
         let n_layers = load_meta(&index_dir, "distance")?.n_layers;
         let matrices = (0..n_layers)
             .filter_map(|l| {
-                layer_dir(&index_dir, l).join("counts").exists()
+                self.layer_dir(part, l).join("counts").exists()
                     .then(|| open_data(&index_dir, l).map_err(|e| olm_to_sk(e, "distance")))
             })
             .collect::<SKResult<Vec<_>>>()?;
@@ -28,14 +26,14 @@ impl KmerPartition {
     /// Open all presence matrices for partition `part`, one per layer.
     /// Layers without a `presence/` directory are skipped.
     pub fn presence_store(&self, part: usize) -> SKResult<LayeredStore<PersistentBitMatrix>> {
-        let index_dir = self.part_dir(part).join(INDEX_SUBDIR);
+        let index_dir = self.index_dir(part);
         if !index_dir.exists() {
             return Ok(LayeredStore::new(vec![]));
         }
         let n_layers = load_meta(&index_dir, "distance")?.n_layers;
         let matrices = (0..n_layers)
             .filter_map(|l| {
-                layer_dir(&index_dir, l).join("presence").exists()
+                self.layer_dir(part, l).join("presence").exists()
                     .then(|| open_data(&index_dir, l).map_err(|e| olm_to_sk(e, "distance")))
             })
             .collect::<SKResult<Vec<_>>>()?;
diff --git a/src/obikpartitionner/src/dump_layer.rs b/src/obikpartitionner/src/dump_layer.rs
index 15252448..46e93cda 100644
--- a/src/obikpartitionner/src/dump_layer.rs
+++ b/src/obikpartitionner/src/dump_layer.rs
@@ -1,14 +1,12 @@
 use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
 use obiskio::{SKError, SKResult, UnitigFileReader};
-use obilayeredmap::{layer_dir, IndexMode, MphfLayer, OLMError};
+use obilayeredmap::{IndexMode, MphfLayer, OLMError};
 use obilayeredmap::meta::PartitionMeta;
 
 use crate::filter::{KmerFilter, passes_all};
 use crate::partition::KmerPartition;
 
-const INDEX_SUBDIR: &str = "index";
-
 fn olm_to_sk(e: OLMError) -> SKError {
     match e {
         OLMError::Io(e) => SKError::Io(e),
@@ -36,7 +34,7 @@ impl KmerPartition {
         filters: &[Box<dyn KmerFilter>],
         mut cb: impl FnMut(CanonicalKmer, Box<[u32]>) -> bool,
     ) -> SKResult<bool> {
-        let index_dir = self.part_dir(part).join(INDEX_SUBDIR);
+        let index_dir = self.index_dir(part);
         if !index_dir.exists() {
             return Ok(true);
         }
@@ -47,7 +45,7 @@ impl KmerPartition {
 
         let mut l = 0;
         loop {
-            let layer_dir = layer_dir(&index_dir, l);
+            let layer_dir = self.layer_dir(part, l);
             if !layer_dir.exists() { break; }
             l += 1;
             let mphf   = MphfLayer::open(&layer_dir, &index_mode).map_err(olm_to_sk)?;
@@ -116,7 +114,7 @@ impl KmerPartition {
         filters: &[Box<dyn KmerFilter>],
         mut cb: impl FnMut(usize, usize, CanonicalKmer, Box<[u32]>) -> bool,
     ) -> SKResult<bool> {
-        let index_dir = self.part_dir(part).join(INDEX_SUBDIR);
+        let index_dir = self.index_dir(part);
         if !index_dir.exists() {
             return Ok(true);
         }
@@ -127,7 +125,7 @@ impl KmerPartition {
 
         let mut layer = 0;
         loop {
-            let layer_dir = layer_dir(&index_dir, layer);
+            let layer_dir = self.layer_dir(part, layer);
             if !layer_dir.exists() { break; }
             let mphf   = MphfLayer::open(&layer_dir, &index_mode).map_err(olm_to_sk)?;
             let reader = UnitigFileReader::open_sequential(&layer_dir.join("unitigs.bin"))?;
diff --git a/src/obikpartitionner/src/index_layer.rs b/src/obikpartitionner/src/index_layer.rs
index 6b48df58..9217df69 100644
--- a/src/obikpartitionner/src/index_layer.rs
+++ b/src/obikpartitionner/src/index_layer.rs
@@ -42,13 +42,13 @@ impl KmerPartition {
         mode: &IndexMode,
         block_bits: u8,
     ) -> Result<usize, SKError> {
-        let part_dir = self.part_dir(i);
-        let dedup_path = part_dir.join("dereplicated.skmer.zst");
+        let partition_dir = self.partition_dir(i);
+        let dedup_path = partition_dir.join("dereplicated.skmer.zst");
         if !dedup_path.exists() {
             return Ok(0);
         }
 
-        let layer_dir = part_dir.join("index").join("layer_0");
+        let layer_dir = self.layer_dir(i, 0);
         if layer_dir.join("mphf.bin").exists() {
             return Ok(0);
         }
@@ -57,14 +57,14 @@ impl KmerPartition {
         let need_counts = filter_active || with_counts;
 
         let mphf1_opt: Option<Mphf> = if need_counts {
-            let p = part_dir.join("mphf1.bin");
+            let p = partition_dir.join("mphf1.bin");
             p.exists().then(|| Mphf::load_full(&p).ok()).flatten()
         } else {
             None
         };
 
         let counts1_opt: Option<PersistentCompactIntVec> = if need_counts {
-            let p = part_dir.join("counts1.bin");
+            let p = partition_dir.join("counts1.bin");
             p.exists()
                 .then(|| PersistentCompactIntVec::open(&p).ok())
                 .flatten()
@@ -125,11 +125,11 @@ impl KmerPartition {
     ///
     /// Deletes `dereplicated.skmer.zst` (+ sidecar), `mphf1.bin`, `counts1.bin`.
     pub fn remove_build_artifacts(&self, i: usize) {
-        let part_dir = self.part_dir(i);
-        let dedup = part_dir.join("dereplicated.skmer.zst");
+        let partition_dir = self.partition_dir(i);
+        let dedup = partition_dir.join("dereplicated.skmer.zst");
         remove_if_exists(&SKFileMeta::sidecar_path(&dedup));
         remove_if_exists(&dedup);
-        remove_if_exists(&part_dir.join("mphf1.bin"));
-        remove_if_exists(&part_dir.join("counts1.bin"));
+        remove_if_exists(&partition_dir.join("mphf1.bin"));
+        remove_if_exists(&partition_dir.join("counts1.bin"));
     }
 }
diff --git a/src/obikpartitionner/src/merge_layer/mod.rs b/src/obikpartitionner/src/merge_layer/mod.rs
index a4046dd2..ea92e3f1 100644
--- a/src/obikpartitionner/src/merge_layer/mod.rs
+++ b/src/obikpartitionner/src/merge_layer/mod.rs
@@ -187,10 +187,6 @@ mod matrix_builder_tests {
     }
 }
 
-// ── helpers ───────────────────────────────────────────────────────────────────
-
-const INDEX_SUBDIR: &str = "index";
-
 // ── KmerPartition::merge_partition ────────────────────────────────────────────
 
 impl KmerPartition {
@@ -212,7 +208,7 @@ impl KmerPartition {
         block_bits: u8,
         evidence: &IndexMode,
     ) -> SKResult<usize> {
-        let dst_index_dir = self.part_dir(i).join(INDEX_SUBDIR);
+        let dst_index_dir = self.index_dir(i);
         if !dst_index_dir.exists() {
             return Ok(0);
         }
@@ -241,7 +237,7 @@ impl KmerPartition {
         // Collect file paths (propagates load_meta errors before the pipeline starts)
         let mut unitig_paths: Vec<PathBuf> = Vec::new();
         for (src, _) in sources.iter() {
-            let src_index_dir = src.part_dir(i).join(INDEX_SUBDIR);
+            let src_index_dir = src.index_dir(i);
             if !src_index_dir.exists() {
                 continue;
             }
@@ -366,7 +362,7 @@ impl KmerPartition {
         {
             let mut col_offset = 0usize;
             for (src, src_n) in sources.iter() {
-                let src_index_dir = src.part_dir(i).join(INDEX_SUBDIR);
+                let src_index_dir = src.index_dir(i);
                 if !src_index_dir.exists() {
                     col_offset += src_n;
                     continue;
diff --git a/src/obikpartitionner/src/partition/kmer_partition.rs b/src/obikpartitionner/src/partition/kmer_partition.rs
index 891e6f8a..ec3a5fa4 100644
--- a/src/obikpartitionner/src/partition/kmer_partition.rs
+++ b/src/obikpartitionner/src/partition/kmer_partition.rs
@@ -22,6 +22,13 @@ use super::count::count_partition;
 use super::dereplicate::{dereplicate_partition, optimal_buckets};
 use super::{PARTITIONS_SUBDIR, SK_EXT};
 
+/// Name of a partition's layered-index subdirectory — the single source of
+/// truth `index_dir`/`layer_dir` build on, replacing what used to be a
+/// `const INDEX_SUBDIR: &str = "index";` (or a bare `"index"` literal)
+/// redefined independently in every module that needed a partition's index
+/// path.
+const INDEX_SUBDIR: &str = "index";
+
 pub struct KmerSpectrum {
     pub f0:     u64,
     pub f1:     u64,
@@ -168,10 +175,22 @@ impl KmerPartition {
     }
 
     /// Path of partition `i` directory.
-    pub fn part_dir(&self, i: usize) -> PathBuf {
+    pub fn partition_dir(&self, i: usize) -> PathBuf {
         self.root_path.join(PARTITIONS_SUBDIR).join(format!("part_{i:05}"))
     }
 
+    /// Path of partition `i`'s layered-index directory (`<partition>/index`).
+    pub fn index_dir(&self, i: usize) -> PathBuf {
+        self.partition_dir(i).join(INDEX_SUBDIR)
+    }
+
+    /// Path of layer `l` within partition `i`'s layered index — composes
+    /// [`index_dir`](Self::index_dir) with `obilayeredmap`'s own
+    /// `layer_N` naming convention rather than reimplementing it.
+    pub fn layer_dir(&self, i: usize, l: usize) -> PathBuf {
+        obilayeredmap::layer_dir(&self.index_dir(i), l)
+    }
+
     pub fn kmer_size(&self) -> usize {
         self.kmer_size
     }
@@ -220,7 +239,7 @@ impl KmerPartition {
         let results: Vec<SKResult<()>> = (0..self.n_partitions)
             .into_par_iter()
             .map(|i| {
-                let dir = self.part_dir(i);
+                let dir = self.partition_dir(i);
                 if !dir.exists() {
                     pb.inc(1);
                     return Ok(());
@@ -268,7 +287,7 @@ impl KmerPartition {
         let results: Vec<SKResult<()>> = (0..self.n_partitions)
             .into_par_iter()
             .map(|i| {
-                let dir = self.part_dir(i);
+                let dir = self.partition_dir(i);
                 let dedup_path = dir.join(format!("dereplicated.{SK_EXT}"));
                 if !dedup_path.exists() {
                     pb.inc(1);
@@ -293,7 +312,7 @@ impl KmerPartition {
         let mut f1: u64 = 0;
 
         for i in 0..self.n_partitions {
-            let path = self.part_dir(i).join("kmer_spectrum_raw.json");
+            let path = self.partition_dir(i).join("kmer_spectrum_raw.json");
             if !path.exists() {
                 continue;
             }
@@ -328,7 +347,7 @@ impl KmerPartition {
 
     fn ensure_writer(&mut self, partition: usize) -> SKResult<&mut SKFileWriter> {
         if self.writers[partition].is_none() {
-            let dir = self.root_path.join(PARTITIONS_SUBDIR).join(format!("part_{:05}", partition));
+            let dir = self.partition_dir(partition);
             fs::create_dir_all(&dir)?;
             let file_path = dir.join(format!("raw.{SK_EXT}"));
             let writer = SKFileWriter::create_with(file_path, Format::Zstd, self.level)?;
diff --git a/src/obikpartitionner/src/query_layer.rs b/src/obikpartitionner/src/query_layer.rs
index c2827e0d..3c8cbc6a 100644
--- a/src/obikpartitionner/src/query_layer.rs
+++ b/src/obikpartitionner/src/query_layer.rs
@@ -4,13 +4,11 @@ use std::path::Path;
 use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
 use obiskio::{SKError, SKResult};
-use obilayeredmap::{layer_dir, IndexMode, MphfLayer, OLMError};
+use obilayeredmap::{IndexMode, MphfLayer, OLMError};
 use obilayeredmap::meta::PartitionMeta;
 
 use crate::partition::KmerPartition;
 
-const INDEX_SUBDIR: &str = "index";
-
 fn olm_to_sk(e: OLMError) -> SKError {
     match e {
         OLMError::Io(io_err) => SKError::Io(io_err),
@@ -175,14 +173,14 @@ impl KmerPartition {
             return Ok(stats);
         }
 
-        let index_dir = self.part_dir(part_idx).join(INDEX_SUBDIR);
+        let index_dir = self.index_dir(part_idx);
         if !index_dir.exists() {
             return Ok(stats);
         }
 
         let meta = PartitionMeta::load(&index_dir).map_err(olm_to_sk)?;
         let layers: Vec<QueryLayer> = (0..meta.n_layers)
-            .map(|i| QueryLayer::open(&layer_dir(&index_dir, i), with_counts, &meta.mode))
+            .map(|i| QueryLayer::open(&self.layer_dir(part_idx, i), with_counts, &meta.mode))
             .collect::<SKResult<_>>()?;
 
         // ── Stage 1: MPHF-only pass, bucket hits by (layer_idx, slot) ────────
diff --git a/src/obikpartitionner/src/rebuild_layer.rs b/src/obikpartitionner/src/rebuild_layer.rs
index d4dc5171..3d23915e 100644
--- a/src/obikpartitionner/src/rebuild_layer.rs
+++ b/src/obikpartitionner/src/rebuild_layer.rs
@@ -17,8 +17,6 @@ use crate::graph_pipeline::materialize_layer;
 use crate::merge_layer::{MergeMode, SrcLayerData};
 use crate::partition::KmerPartition;
 
-const INDEX_SUBDIR: &str = "index";
-
 // ── Builders — pair matrix builder + column builders for one mode ─────────────
 
 enum Builders {
@@ -193,7 +191,7 @@ impl KmerPartition {
         n_genomes: usize,
         block_bits: u8,
     ) -> SKResult<()> {
-        let src_index_dir = src.part_dir(i).join(INDEX_SUBDIR);
+        let src_index_dir = src.index_dir(i);
         if !src_index_dir.exists() {
             return Ok(());
         }
@@ -214,8 +212,8 @@ impl KmerPartition {
         }
 
         // ── Build MPHF in dst layer_0 ─────────────────────────────────────────
-        let dst_index_dir = self.part_dir(i).join(INDEX_SUBDIR);
-        let dst_layer_dir = dst_index_dir.join("layer_0");
+        let dst_index_dir = self.index_dir(i);
+        let dst_layer_dir = self.layer_dir(i, 0);
 
         let n_new = materialize_layer(g, &dst_layer_dir, block_bits, &IndexMode::Exact)?;
         let dst_mphf = MphfLayer::open(&dst_layer_dir, &IndexMode::Exact)
diff --git a/src/obikpartitionner/src/select_layer.rs b/src/obikpartitionner/src/select_layer.rs
index 847d1a4a..691dba55 100644
--- a/src/obikpartitionner/src/select_layer.rs
+++ b/src/obikpartitionner/src/select_layer.rs
@@ -8,13 +8,11 @@ use obicompactvec::{
     PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder,
 };
 use obilayeredmap::meta::PartitionMeta;
-use obilayeredmap::{layer_dir, OLMError};
+use obilayeredmap::OLMError;
 use obiskio::{SKError, SKResult};
 
 use crate::partition::KmerPartition;
 
-const INDEX_SUBDIR: &str = "index";
-
 // ── AggOp ─────────────────────────────────────────────────────────────────────
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -147,7 +145,7 @@ impl KmerPartition {
         output_presence: bool,
         in_place: bool,
     ) -> SKResult<()> {
-        let src_index_dir = src.part_dir(i).join(INDEX_SUBDIR);
+        let src_index_dir = src.index_dir(i);
         if !src_index_dir.exists() {
             return Ok(());
         }
@@ -157,7 +155,7 @@ impl KmerPartition {
             return Ok(());
         }
 
-        let dst_index_dir = self.part_dir(i).join(INDEX_SUBDIR);
+        let dst_index_dir = self.index_dir(i);
         if !in_place {
             fs::create_dir_all(&dst_index_dir)?;
         }
@@ -165,10 +163,10 @@ impl KmerPartition {
         let data_subdir = if output_presence { "presence" } else { "counts" };
 
         for l in 0..src_meta.n_layers {
-            let src_layer_dir = layer_dir(&src_index_dir, l);
+            let src_layer_dir = src.layer_dir(i, l);
             if !src_layer_dir.exists() { continue; }
 
-            let dst_layer_dir = layer_dir(&dst_index_dir, l);
+            let dst_layer_dir = self.layer_dir(i, l);
 
             let counts_dir   = src_layer_dir.join("counts");
             let presence_dir = src_layer_dir.join("presence");
diff --git a/src/obikphylo/src/siblings/build.rs b/src/obikphylo/src/siblings/build.rs
index b2760c39..28c0768b 100644
--- a/src/obikphylo/src/siblings/build.rs
+++ b/src/obikphylo/src/siblings/build.rs
@@ -7,7 +7,7 @@ use rayon::prelude::*;
 use obikpartitionner::KmerPartition;
 use obipipeline::ThrottleGuard;
 use obikseq::CanonicalKmer;
-use obilayeredmap::{layer_dir, MphfLayer};
+use obilayeredmap::MphfLayer;
 use obilayeredmap::meta::PartitionMeta;
 use obisys::progress_bar;
 
@@ -16,7 +16,7 @@ use obikindex::KmerIndex;
 
 use super::cache::PartitionCache;
 use super::helpers::{central_base, is_minorant};
-use super::{olm_to_ok, FamilyMask, SiblingAnnexBuilder, ANNEX_FILE_NAME, INDEX_SUBDIR};
+use super::{olm_to_ok, FamilyMask, SiblingAnnexBuilder, ANNEX_FILE_NAME};
 
 // ── obipipeline data types ─────────────────────────────────────────────────
 
@@ -92,7 +92,7 @@ impl SiblingAnnexBuildExt for KmerIndex {
         let pb = progress_bar("sibling_annex", n_parts as u64, "partitions");
         let mut total_slots: u64 = 0;
         for part in 0..n_parts {
-            let index_dir = self.partition().part_dir(part).join(INDEX_SUBDIR);
+            let index_dir = self.partition().index_dir(part);
             if !index_dir.exists() {
                 pb.inc(1);
                 continue;
@@ -101,7 +101,7 @@ impl SiblingAnnexBuildExt for KmerIndex {
 
             let mut part_slots: u64 = 0;
             for l in 0..meta.n_layers {
-                part_slots += build_layer_sibling_annex(self, &layer_dir(&index_dir, l), n_parts, l, &cache)?;
+                part_slots += build_layer_sibling_annex(self, &self.partition().layer_dir(part, l), n_parts, l, &cache)?;
             }
             total_slots += part_slots;
             pb.inc(1);
diff --git a/src/obikphylo/src/siblings/cache.rs b/src/obikphylo/src/siblings/cache.rs
index 00cba141..8ffef9fe 100644
--- a/src/obikphylo/src/siblings/cache.rs
+++ b/src/obikphylo/src/siblings/cache.rs
@@ -5,14 +5,14 @@ use std::path::Path;
 use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix, PersistentSparseBitMatrix};
 use obikpartitionner::KmerPartition;
 use obikseq::CanonicalKmer;
-use obilayeredmap::{layer_dir, Layer, OLMResult};
+use obilayeredmap::{Layer, OLMResult};
 use obilayeredmap::meta::{IndexMode, PartitionMeta};
 use obisys::progress_bar;
 
 use obikindex::OKIResult;
 
 use super::iter::SiblingLayerExt;
-use super::{olm_to_ok, SiblingAnnex, INDEX_SUBDIR};
+use super::{olm_to_ok, SiblingAnnex};
 
 /// Every partition's already-open layers, built **once** for the whole
 /// `build_sibling_annex` run and shared (read-only) across every lookup, in
@@ -174,7 +174,7 @@ impl PartitionCache {
         let built: Vec<(Vec<Mat>, usize)> = (0..n_parts)
             .into_par_iter()
             .map(|part| -> OKIResult<(Vec<Mat>, usize)> {
-                let index_dir = partition.part_dir(part).join(INDEX_SUBDIR);
+                let index_dir = partition.index_dir(part);
                 if !index_dir.exists() {
                     pb.inc(1);
                     return Ok((Vec::new(), 0));
@@ -182,7 +182,7 @@ impl PartitionCache {
                 let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
                 let mut mats = Vec::with_capacity(meta.n_layers);
                 for l in 0..meta.n_layers {
-                    let Ok(mat) = Mat::open(&layer_dir(&index_dir, l), &meta.mode, with_counts) else { continue };
+                    let Ok(mat) = Mat::open(&partition.layer_dir(part, l), &meta.mode, with_counts) else { continue };
                     mats.push(mat);
                 }
                 pb.inc(1);
diff --git a/src/obikphylo/src/siblings/family_scan.rs b/src/obikphylo/src/siblings/family_scan.rs
index 227a5504..0215bc68 100644
--- a/src/obikphylo/src/siblings/family_scan.rs
+++ b/src/obikphylo/src/siblings/family_scan.rs
@@ -54,7 +54,6 @@ use std::sync::atomic::{AtomicU8, Ordering};
 use rayon::prelude::*;
 
 use obikseq::CanonicalKmer;
-use obilayeredmap::layer_dir;
 use obilayeredmap::meta::PartitionMeta;
 use obipipeline::{ThrottleGuard, throttle};
 
@@ -64,7 +63,7 @@ use obikindex::KmerIndex;
 use super::cache::{Mat, PartitionCache};
 use super::helpers::central_base;
 use super::iter::SiblingEntry;
-use super::{olm_to_ok, FamilyMask, SiblingAnnex, ANNEX_FILE_NAME, INDEX_SUBDIR};
+use super::{olm_to_ok, FamilyMask, SiblingAnnex, ANNEX_FILE_NAME};
 
 /// Families per batch — see the module docs for the memory-vs-per-partition-
 /// density trade-off this picks a point on. At ~90 genomes and a few
@@ -106,13 +105,13 @@ pub(crate) fn sibling_layer_dirs(index: &KmerIndex) -> OKIResult<Vec<PathBuf>> {
     let n_parts = index.n_partitions();
     let mut layer_dirs = Vec::new();
     for part in 0..n_parts {
-        let index_dir = index.partition().part_dir(part).join(INDEX_SUBDIR);
+        let index_dir = index.partition().index_dir(part);
         if !index_dir.exists() {
             continue;
         }
         let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
         for l in 0..meta.n_layers {
-            let this_layer_dir = layer_dir(&index_dir, l);
+            let this_layer_dir = index.partition().layer_dir(part, l);
             let annex_path = this_layer_dir.join(ANNEX_FILE_NAME);
             if !annex_path.exists() {
                 return Err(OKIError::InvalidInput(format!(
diff --git a/src/obikphylo/src/siblings/mod.rs b/src/obikphylo/src/siblings/mod.rs
index 9a1a3078..40458964 100644
--- a/src/obikphylo/src/siblings/mod.rs
+++ b/src/obikphylo/src/siblings/mod.rs
@@ -75,7 +75,6 @@ use obilayeredmap::OLMError;
 
 use obikindex::OKIError;
 
-pub(super) const INDEX_SUBDIR: &str = "index";
 pub(super) const ANNEX_FILE_NAME: &str = "siblings.psib";
 
 pub(super) fn olm_to_ok(e: OLMError) -> OKIError {
diff --git a/src/obikphylo/src/siblings/tests.rs b/src/obikphylo/src/siblings/tests.rs
index 988349e2..93784782 100644
--- a/src/obikphylo/src/siblings/tests.rs
+++ b/src/obikphylo/src/siblings/tests.rs
@@ -3,7 +3,6 @@ use std::path::Path;
 
 use obikseq::{CanonicalKmer, Kmer, Sequence};
 use obilayeredmap::MphfLayer;
-use obilayeredmap::layer_dir;
 use obilayeredmap::meta::PartitionMeta;
 use obisys::Reporter;
 use tempfile::tempdir;
@@ -20,7 +19,7 @@ use super::helpers::is_minorant;
 use super::sankoff_bundle::SankoffBundleExt;
 use super::stats::SiblingStatsExt;
 use super::subsample::EntropyBias;
-use super::{FamilyMask, SiblingAnnex, ANNEX_FILE_NAME, INDEX_SUBDIR};
+use super::{FamilyMask, SiblingAnnex, ANNEX_FILE_NAME};
 
 // k must be >= 11 (project constraint, "k ∈ [11,31]"); k=11, level_max=1,
 // theta=0.0 mirror `obiskbuilder`'s own tests (smaller k/level_max
@@ -89,10 +88,10 @@ fn canonical(ascii: &[u8]) -> CanonicalKmer {
 /// Read back the annex entry for a given canonical k-mer from the merged
 /// index's (single) partition/layer, asserting it was found at all.
 fn annex_info_for(idx: &KmerIndex, kmer: CanonicalKmer) -> FamilyMask {
-    let index_dir = idx.partition().part_dir(0).join(INDEX_SUBDIR);
+    let index_dir = idx.partition().index_dir(0);
     let meta = PartitionMeta::load(&index_dir).unwrap();
     for l in 0..meta.n_layers {
-        let layer_dir = layer_dir(&index_dir, l);
+        let layer_dir = idx.partition().layer_dir(0, l);
         let mphf = MphfLayer::open(&layer_dir, &meta.mode).unwrap();
         if let Some(slot) = mphf.find(kmer) {
             let annex = SiblingAnnex::open(&layer_dir.join(ANNEX_FILE_NAME)).unwrap();
@@ -168,7 +167,7 @@ fn sibling_annex_works_after_pack_sparse() {
     let merged = merge_two(dir.path(), &g1, &g2);
     merged.pack_matrices(true).expect("pack_matrices(sparse)");
 
-    let index_dir = merged.partition().part_dir(0).join(INDEX_SUBDIR);
+    let index_dir = merged.partition().index_dir(0);
     assert!(
         index_dir.join("layer_0").join("presence").join("is_multi.prsb").exists(),
         "pack_matrices(true) must leave the sparse marker file behind"
@@ -311,10 +310,10 @@ fn sibling_annex_no_empty_masks_after_build() {
     let g1 = build_single_genome_index(dir.path(), "g1", &seq);
     g1.build_sibling_annex().expect("build_sibling_annex");
 
-    let index_dir = g1.partition().part_dir(0).join(INDEX_SUBDIR);
+    let index_dir = g1.partition().index_dir(0);
     let meta = PartitionMeta::load(&index_dir).expect("partition meta");
     for l in 0..meta.n_layers {
-        let layer_dir = layer_dir(&index_dir, l);
+        let layer_dir = g1.partition().layer_dir(0, l);
         let annex = SiblingAnnex::open(&layer_dir.join(ANNEX_FILE_NAME)).expect("annex open");
         for slot in 0..annex.len() {
             let mask = annex.get(slot).expect("slot must have an entry");
@@ -362,7 +361,7 @@ fn sibling_annex_records_the_real_layer_of_each_family_member() {
     let merged = merge_two(dir.path(), &g1, &g2);
     merged.build_sibling_annex().expect("build_sibling_annex");
 
-    let index_dir = merged.partition().part_dir(0).join(INDEX_SUBDIR);
+    let index_dir = merged.partition().index_dir(0);
     let meta = PartitionMeta::load(&index_dir).unwrap();
     assert_eq!(meta.n_layers, 2, "fixture assumption: one merge, one new layer");