diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 50860cb5..84bd25e3 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -57,6 +57,13 @@ see `package.json` for `test:integration`, `test:coverage`, and friends.
   description states a number, cite the benchmark that produced it (see
   [docs/performance-envelopes.md](docs/performance-envelopes.md) for the
   pattern). Don't state an estimate as if it were measured.
+- **Measurements carry numbers, not provenance.** Public commit messages and
+  docs give the SHAPE a number was taken at and never where it was taken: no
+  hostnames, no store or deployment identities, no operational anecdotes about
+  someone's running system. "A 14,056-noun / 72,679-verb production-shaped
+  store, measured solo under an exclusive lock" tells a reader everything the
+  number depends on; the machine it ran on and whose data it was tell them
+  nothing except where somebody's infrastructure lives.
 
 ## License