diff --git a/docs/formats/core-data-regions.md b/docs/formats/core-data-regions.md
index ab7f5ca..5d1d65d 100644
--- a/docs/formats/core-data-regions.md
+++ b/docs/formats/core-data-regions.md
@@ -875,6 +875,19 @@ This does not change the earlier conclusion that 63 of 64 headers imply one
 stride -- that remains 63 of 64 -- but the mapping is not a bijection and
 should not be used as one.
 
+**The header does not encode the stride.** All 90 two-byte windows of
+`header[0..14]` were tested: none maps to a unique stride across every group.
+So there is no simple two-byte field to read the record size from, and the
+stride must come from the offset table, as the parser already does, by
+differencing the first two tagged offsets.
+
+That is the right implementation anyway, and this makes it more than a
+convenience: given three chunks sharing one header with three different
+strides, any dispatch that assumed "header implies size" would be wrong for
+1,123 chunks' worth of type coverage on at least one layout. Reading the
+stride from the offsets is the only interpretation consistent with all the
+measurements.
+
 **The ASCII reading is confirmed as a type tag.** Grouping by header bytes 9
 and 13: