diff --git a/README.md b/README.md
index 0cb3542..ad1058a 100644
--- a/README.md
+++ b/README.md
@@ -1235,6 +1235,13 @@ capability alters those two columns in a migration of its own.
 There is no clock here. Nothing retires on its own or on a schedule, no
 call takes a duration, and when a chart should go is the host's policy.
 
+On Postgres a retirement and the creates and migrations on its chart are
+ordered by a per-chart advisory lock.
+[What the chart lock reaches](https://github.com/riddler/statifier_persistence/blob/main/docs/chart-lock.md) says what waits on what, why the
+lock's key is scoped by the store and not by a tenant, the one way a
+transaction of your own can deadlock on it, and what it means for test
+suites running in parallel.
+
 ## Pruning finished executions
 
 A finished execution keeps its last position blob and, on an adapter that
diff --git a/docs/chart-lock.md b/docs/chart-lock.md
new file mode 100644
index 0000000..26810e2
--- /dev/null
+++ b/docs/chart-lock.md
@@ -0,0 +1,192 @@
+# What the chart lock reaches
+
+Retiring a chart (`StatifierPersistence.Executions.retire_chart/4`) and
+putting an execution on a chart race each other: a create or a migration
+reads the chart's tombstone, finds none, and writes its execution, while
+a retirement counts the chart's pins and writes the tombstone. On the
+Ecto adapter over Postgres a per-chart advisory lock orders the two, so
+one of them always sees the other. This page says what that lock makes
+wait, what its key is, the one way your own code can deadlock on it, and
+what it means for test suites running in parallel.
+
+The design is ADR-0012's 2026-09-28 Amendment (the lock) and its
+2026-09-29 Amendment (the key). The lock shipped in 0.23.0. The key this
+page describes, scoped by the store, ships in the first release after
+0.23.1; 0.23.0 and 0.23.1 key the lock on the content hash alone, as
+"Before the store-scoped key" below describes.
+
+## Where the lock is taken
+
+Only `StatifierPersistence.Storage.Ecto`, and only when the repo's
+adapter is Postgres, takes it. The in-memory adapter checks the tombstone
+inside the same Agent transition that writes, and needs no lock. An Ecto
+repo on another backend takes none, and the race is narrowed there, not
+closed.
+
+The lock is transaction-scoped (`pg_advisory_xact_lock` and
+`pg_advisory_xact_lock_shared`). It is held until the transaction that
+took it commits or rolls back, and a statement outside any transaction
+releases it when the statement ends.
+
+## The two modes
+
+- **Shared**, taken by every tombstone read: `Storage.check_chart_retired/2`
+  and the adapter's `fetch_retired_info/2`. `Executions.create/4`,
+  `Executions.migrate/4` and `Executions.migrate_tree/4` each make that
+  read inside the transaction they write in (for a tree, once per node's
+  `to` chart), so under the default serialization strategy they hold the
+  shared lock until the execution they put on the chart has committed.
+- **Exclusive**, taken by a retirement: the adapter's `retire_chart/3`
+  takes it as the first statement of its transaction, before it counts
+  anything, and holds nothing on the chart before it.
+
+A `serialization:` strategy of your own opens no database transaction
+for the lock to live in, so a create under it, outside a transaction of
+yours, holds the shared lock only for the read's own statement. The
+race is narrowed there, not closed.
+
+## What waits on what
+
+For one chart in one store (the next section says what "one store" is):
+
+| Already holding | Arriving | Waits? | What the arriving call then answers |
+|---|---|---|---|
+| a tombstone read (shared) | a tombstone read, from any create, migration or check | no | its own read; any number of writers on one chart proceed together |
+| a tombstone read (shared) | a retirement (exclusive) | yes, until the reader's transaction ends | if the reader committed an execution on the chart, the retirement sees it and refuses with `{:error, {:pinned, counts}}`; otherwise it decides as it would have |
+| a retirement (exclusive) | a tombstone read (shared) | yes, until the retirement's transaction ends | the read sees the tombstone, and the create or migration answers `{:error, {:chart_retired, info}}` having written nothing |
+| a retirement (exclusive) | another retirement (exclusive) | yes | the retired arm if the first one tombstoned the chart, never a second tombstone |
+
+Nothing waits across two different charts, or across two different
+stores, short of a collision in the key's 32-bit hash, where two keys
+behave as one. The per-execution lock the default
+serialization strategy takes (`lock_execution/3`) is a different key
+space and never meets this one.
+
+Only the losing interleaving changes what a call answers. A create that
+does not race a retirement answers what it always answered.
+
+## The key, and why its unit is the store
+
+The key is two `int4`s:
+
+    (namespace, hashtext(store <> " " <> content_hash))
+
+The namespace is a constant of the adapter (`@chart_lock_namespace`, the
+bytes `SPCH`). The store is the table of your host module's chart schema,
+under its Postgres schema prefix (the `:prefix` option, Ecto's
+`@schema_prefix`) when it has one, each part written as a double-quoted
+identifier: `"statifier_charts"` with the default table name and no
+prefix, or `"loans"."statifier_charts"` with `prefix: "loans"`. The
+`:table_prefix` option is part of the table name, so it is in the key
+through the name.
+
+So:
+
+- **Two stores in one database do not wait on each other.** Two table
+  names (from two `:table_prefix` values or two `:tables` overrides), or
+  one table name under two schema prefixes, give one content hash two
+  keys.
+- **Two host modules on one physical table are one store.** Same table,
+  same prefix: they share the key, because they share the rows the lock
+  guards.
+- **Tenants sharing one charts table share the lock.** The charts table's
+  unique index is on `content_hash` alone. A tenant column you add with
+  `:leading_columns` goes on the table, not into that index, so a chart
+  has one row, and one tombstone, per store. A lock keyed per tenant
+  would let a create for one tenant read the chart as not retired while
+  a retirement for another tenant tombstoned that same row, which is the
+  race the lock exists to close. The consequence is mild: creates never
+  wait on each other (shared never waits on shared), and a retirement
+  waits only for in-flight writes on the one chart it retires.
+
+### The prefix-free edge
+
+A chart schema with no schema prefix, which is the default, is keyed by
+its table name alone. Which Postgres schema that name resolves to is the
+connection's `search_path`, and the key does not include it. Two hosts
+in one database with the same chart table name, no prefix, and
+different `search_path` settings share a key though their rows are
+apart. They wait on each other exactly as every store in one database
+did before the store-scoped key; nothing answers differently. Give each
+its own `:prefix` if the waits matter.
+
+### Before the store-scoped key
+
+On 0.23.0 and 0.23.1 the key is `(namespace, hashtext(content_hash))`:
+database-wide. Every store in one database shares the lock of every
+content hash they have in common, whatever tables or prefixes they use.
+
+## Reading a chart, then retiring it, in one transaction of your own
+
+This is the one way the lock can deadlock, and no call in this package
+does it on its own: `Executions.retire_chart/4` reads no tombstone before
+the adapter's `retire_chart/3` takes the exclusive lock as the first
+statement of its transaction, and the creates, migrations and checks
+take only the shared lock.
+
+Your own transaction can. If it first reads a chart's tombstone, or
+creates or migrates an execution onto that chart, and then retires the
+same chart, it asks for the exclusive lock while it still holds the
+shared one. One such transaction alone is granted the upgrade. Two at
+once, on one chart in one store, each hold the shared lock and each wait
+for the other to let it go, and Postgres ends one of them with
+`deadlock_detected` (SQLSTATE `40P01`).
+
+"Your own transaction" includes more than an explicit
+`Repo.transaction/2`:
+
+- an `Ecto.Multi`, or any function of yours that runs inside one;
+- this package's calls made inside a transaction you opened, because
+  they join it (README, "Writing inside a caller's transaction");
+- **a test under the Ecto SQL sandbox**, where the whole test is one
+  transaction, so a test that creates an execution on a chart and later
+  retires that chart is such a caller.
+
+To find one in your code, list the files that retire a chart and, in
+each, every call that reads a tombstone or puts an execution on a chart:
+
+    grep -rlE 'retire_chart' lib test \
+      | xargs grep -nE 'retire_chart|check_chart_retired|fetch_retired_info|create\(|migrate\(|migrate_tree\('
+
+For each file, ask whether a read and a retirement of the same chart can
+run inside one transaction, and whether two such transactions can run at
+once. If they can, any one of these removes the deadlock:
+
+- retire in a transaction of its own, outside the one that read;
+- retire first: a transaction that takes the exclusive lock before it
+  reads holds nothing it has to upgrade;
+- in tests, give each concurrently running test its own content hashes,
+  or run those tests with `async: false`.
+
+## Test suites in parallel
+
+`StatifierPersistence.Testing.StorageConformance` gives every chart a
+case retires a suffix derived from the using module's name. Two modules
+running the suite at once against one database retire different content
+hashes, so they never meet on one key, whatever their stores.
+
+With the store-scoped key, two suites in one database meet on the lock,
+for a hash they have in common, only when they also share a charts
+table: two host modules on the same table and prefix. Suites on
+different tables or prefixes never wait on each other's charts.
+
+What neither covers is two operating-system processes running the same
+module against the same database: two checkouts of one project, or
+`mix test --partitions` with every partition pointed at one database.
+The module name, and so the suffix, is the same in both, and so are the
+table and the key. Give each process its own test database: the
+database name in `config/test.exs` is where to do it (this package's
+own suite reads `PGDATABASE`). Making this package's own default test
+database name per checkout is a later item.
+
+## The router and the Oban adapter do not choose your version
+
+`statifier_router` and `statifier_oban` depend on this package through a
+version requirement (the Oban adapter's is optional). Each keeps its own
+`mix.lock` for its own test suite, and that file never reaches your
+application: the version of
+`statifier_persistence` you run, and so which key the lock takes, is the
+one your application's `mix.lock` resolved. To take the store-scoped key
+once it is released, update `statifier_persistence` in your own project
+(`mix deps.update statifier_persistence`) and check the version it
+lands on in your `mix.lock`.
diff --git a/docs/retention.md b/docs/retention.md
index c890657..2b1317c 100644
--- a/docs/retention.md
+++ b/docs/retention.md
@@ -147,6 +147,11 @@ example a replay tool, decide which executions it may read from their
 | The input log or position of an execution that has not finished | The execution can still take a step. With its position gone, the step fails with `:execution_position_missing`. With its log gone, the next input is written at ordinal 0 again, and the log reads as if the execution had started there. |
 | Some, but not all, of an execution's input log rows | A log with a gap looks complete and is not. A replay built from it replays a different execution. |
 
+Retiring a chart on Postgres takes a per-chart advisory lock that the
+creates and migrations on that chart also take.
+[What the chart lock reaches](https://github.com/riddler/statifier_persistence/blob/main/docs/chart-lock.md) says what it makes wait and how to
+avoid a deadlock on it inside a transaction of your own.
+
 ## What pruning does not do
 
 - **It keeps the answer and the metadata.** `outcome_blob`, `failure` and