diff --git a/preconfig b/preconfig
index 5b9fb5863..c8ab004a6 100755
--- a/preconfig
+++ b/preconfig
@@ -447,10 +447,14 @@ GRAPHVIZDIR=${GRAPHVIZDIR:=" "}
 CXX_EXT=${CXX_EXT:=cpp}
 F77_EXT=${F77_EXT:=f}
 F90_EXT=${F90_EXT:=f90}
+#
+# Default extension for executable binaries. The default for unix systems
+# is the empty field. For windows systesms and cygwin, you should put
+# ".exe" .
+#
 EXE_EXT=${EXE_EXT:=}
 
 
-
 CT_SHARED_LIB=${CT_SHARED_LIB:=clib}
 
 # The font to use in reaction path diagrams. This must be a font name