diff --git a/Capabilities.md b/Capabilities.md
index b8135ab..ddaa6d5 100644
--- a/Capabilities.md
+++ b/Capabilities.md
@@ -20,6 +20,8 @@ but updated to take into account literal caps and immutable directories:
 11: mutable directory read-only capability string         URI:DIR2-RO:
 12: mutable directory verify capability string            URI:DIR2-Verifier:
 
+13: unknown/future capability strings                     <anything else>
+
 In Tahoe, directories are built out of mutable files (a directory is really
 just a particular way to interpret the contents of a given mutable file), and
 non-directory mutable files aren't used very much. All normal data files are