diff --git a/zscript-doc.md b/zscript-doc.md
index 34eeb33..f983035 100644
--- a/zscript-doc.md
+++ b/zscript-doc.md
@@ -72,6 +72,14 @@ class Name [: BaseClass] [Class flags...];
 [Class content...]
 ```
 
+If the class is defined within the same archive as the current file, then one can continue a class definition with the syntax:
+
+```
+extend class Name
+```
+
+In place of the class header.
+
 Class flags
 -----------