diff --git a/Doc/ref/ref4.tex b/Doc/ref/ref4.tex
index e2bfcca..7176f20 100644
--- a/Doc/ref/ref4.tex
+++ b/Doc/ref/ref4.tex
@@ -25,6 +25,12 @@
 The expression read and evaluated by the built-in function
 \function{input()} is a code block.
 
+A code block is executed in an \dfn{execution
+frame}\indexii{execution}{frame}.  A frame contains some
+administrative information (used for debugging) and determines where
+and how execution continues after the code block's execution has
+completed.
+
 A \dfn{scope}\index{scope} defines the visibility of a name within a
 block.  If a local variable is defined in a block, it's scope includes
 that block.  If the definition occurs in a function block, the scope