diff --git a/Modules/CMakeCXXCompilerId.cpp b/Modules/CMakeCXXCompilerId.cpp
index f1331474ae..87baea273d 100644
--- a/Modules/CMakeCXXCompilerId.cpp
+++ b/Modules/CMakeCXXCompilerId.cpp
@@ -1,3 +1,6 @@
+/* This source file must have a .cpp extension so that all C++ compilers
+   recognize the extension without flags.  Borland does not know .cxx for
+   example.  */
 #ifndef __cplusplus
 # error "A C compiler has been selected for C++."
 #endif