diff --git a/64.md b/64.md
index 3bc09b25..0293c6a4 100644
--- a/64.md
+++ b/64.md
@@ -35,3 +35,7 @@ tbd.
 ## Client Behavior
 
 tbd.
+
+## Additional info
+- [PGN Specification](https://github.com/mliebelt/pgn-spec-commented/blob/main/pgn-specification.md) PGN (Portable Game Notation) specification
+- [PGN Specification Supplement](https://github.com/mliebelt/pgn-spec-commented/blob/main/pgn-spec-supplement.md) Addition for adding graphical elements, clock values, eval, ...