diff --git a/canonical-binary.md b/canonical-binary.md
index f6b5462..b771970 100644
--- a/canonical-binary.md
+++ b/canonical-binary.md
@@ -24,8 +24,9 @@ binary syntax.
 Streaming formats ("format C") *MUST NOT* be used.
 Annotations *MUST NOT* be present.
 Placeholders *MUST NOT* be used.
-Where possible, fixed-length ("format A") *MUST* be used in preference
-to variable-length ("format B") formats.
+Whenever there is a choice between fixed-length ("format A") or
+variable-length ("format B") formats, the fixed-length format *MUST* be
+used.
 
 **Sets.**
 The elements of a `Set` *MUST* be serialized sorted in ascending order