diff --git a/docs/architecture/anniversary-matching.md b/docs/architecture/anniversary-matching.md
index 19c93a7..bd05d09 100644
--- a/docs/architecture/anniversary-matching.md
+++ b/docs/architecture/anniversary-matching.md
@@ -28,8 +28,13 @@ IANA timezone, defaulting to `Europe/Berlin` in the matcher.
 # Leap-Day Behavior
 
 A February 29 post matches February 29 when the current year is a leap year.
-In a non-leap year it matches March 1, preserving one anniversary event for
-leap-day posts.
+In a non-leap year it matches March 1, but only when the post's year is
+exactly ten years before the current year (`today.year − 10`). Because
+`today.year − 10` is never itself a leap year whenever today is a leap year
+(`10 mod 4 == 2`), the Feb 29 ↔ Feb 29 branch is reached independently of
+the year-equality check: any Feb 29 post from a prior leap year is a
+candidate, and its canonical URL keeps the original `YYYY/02/29/<slug>/`
+path.
 
 # Output and Integration