diff --git a/Source/Makefile.in b/Source/Makefile.in
index ccfe28c92..73be7ff55 100644
--- a/Source/Makefile.in
+++ b/Source/Makefile.in
@@ -49,6 +49,8 @@ LEX = flex
 LEXFLAGS =
 YACC = bison
 
+# GNUSTEP_INSTALL_PREFIX must be defined here and not in config.h because
+# the installing person may set it on the `make' command line.
 DEFS = -DGNUSTEP_INSTALL_PREFIX=$(prefix) @DEFS@
 
 # File name extensions