diff --git a/docs/stack.md b/docs/stack.md
index e53a760..2ee6b55 100644
--- a/docs/stack.md
+++ b/docs/stack.md
@@ -33,6 +33,8 @@ When an indirect atom is copied from one stack frame to another, its size is rep
 forwarding pointer. When a cell is copied from one stack frame to another, its
 head is replaced with a forwarding pointer.
 
+The tag for a forwarding pointer is 111.
+
 ## Memory Layout
 
 Computation proceeds in a memory arena, using two stacks growing towards each other from opposite ends.