diff --git a/scans/README.md b/scans/README.md
index 359beea..172b453 100644
--- a/scans/README.md
+++ b/scans/README.md
@@ -30,7 +30,7 @@ Details of the files:
 * trysys.s: page 65  of 01-s1.pdf
 * scope.v: pages 3-18 of 03-scope.pdf
 * cas.s: pages 1-8 of 04-cas.pdf
-* cas.x: pages 9-17 of 04-cas.pdf
+* cas.cas: pages 9-17 of 04-cas.pdf
 * adm.s: pages 2-11  of 05-1-4.pdf
 * ald.s: pages 13-17 of 05-1-4.pdf
 * apr.s: pages 19-30 of 05-1-4.pdf
diff --git a/scans/cas.x b/scans/cas.cas
similarity index 100%
rename from scans/cas.x
rename to scans/cas.cas
diff --git a/src/cmd/cas.x b/src/cmd/cas.cas
similarity index 100%
rename from src/cmd/cas.x
rename to src/cmd/cas.cas