diff --git a/spec/ledger.md b/spec/ledger.md
index dfee2ae..16c9022 100644
--- a/spec/ledger.md
+++ b/spec/ledger.md
@@ -132,10 +132,11 @@ how the books are kept.
 
 ### BS-ledger-13: Audits and attestations (B-45)
 
-- **Status:** reserved
-- **Statement:** Reserved: allocated in the wanderbooks plan (8.0.3), not yet stated in `docs/spec/rules.md`.
+- **Status:** CONVENTION
+- **Statement:** An audit of the books through a day MUST give the same findings for the same books and programme; a claim that someone checked MUST rest on the books as they were when the audit ran, MUST NOT count when what it read disagrees with the books, and its method is the claimant's own word.
 - **Sources:**
-  - none yet
-- **Symbolic form:** none yet.
-- **Examples:** none yet
-- **Applicability:** none yet.
+  - external: Revenue Procedure 98-25, section 5.01, machine-sensible records with an audit trail
+  - vendored `sources/irs-records.md`: "In addition, you must keep supporting documents"
+- **Symbolic form:** `findings(books, through) = f(books, through, programme)`; `counted(a, bar) = meets(method(a), bar) and not disagrees(a)`.
+- **Examples:** none yet. The example schema has no audit event or query; an audit example waits on a schema extension.
+- **Applicability:** engines that are audited.
diff --git a/spec/rules-index.md b/spec/rules-index.md
index 8d30d5f..586f39f 100644
--- a/spec/rules-index.md
+++ b/spec/rules-index.md
@@ -6,8 +6,8 @@ what its statement rests on and the examples that demonstrate it.
 
 A rule whose only source is a human choice carries rules.md's **CHOICE**, bookspec's status of the same name:
 the specification names the options, an engine declares its pick
-(`claims.choices`, `bookspec_checker::Claims`). Rules B-39, B-40, B-43
-and B-45 are allocated in `plan/served.md` 8.0.3 and not yet stated: their
+(`claims.choices`, `bookspec_checker::Claims`). Rules B-39, B-40 and B-43
+are allocated in `plan/served.md` 8.0.3 and not yet stated: their
 identifier and area are fixed here so a later unit fills them in
 without moving anyone else's number.
 
@@ -57,4 +57,4 @@ without moving anyone else's number.
 | B-42 | `BS-arap-6` | arap | CONVENTION | A settlement is exact, one party, and names its lines; the gross discount and the gross fee | — |
 | B-43 | `BS-entry-5` | entry | — | Drafts and evidence (allocated, not yet stated) | — |
 | B-44 | `BS-ledger-12` | ledger | CONVENTION | Evidence is read from the bytes, never trusted over them; identical files are kept once | — |
-| B-45 | `BS-ledger-13` | ledger | — | Audits and attestations (allocated, not yet stated) | — |
+| B-45 | `BS-ledger-13` | ledger | CONVENTION | An audit is deterministic over the books; an attestation is a claim by a named client | — |