diff --git a/Source/cmIfCommand.h b/Source/cmIfCommand.h
index fa1df2704..9e5576234 100644
--- a/Source/cmIfCommand.h
+++ b/Source/cmIfCommand.h
@@ -113,9 +113,11 @@ public:
       "will be evaluated. Then NOT operators and finally AND, OR operators "
       "will be evaluated. Possible expressions are:\n"
       "  IF(variable)\n"
-      "True if the variable's value is not empty, 0, FALSE, OFF, or NOTFOUND.\n"
+      "True if the variable's value is not empty, 0, N, NO, OFF, FALSE, "
+      "NOTFOUND, or <variable>-NOTFOUND.\n"
       "  IF(NOT variable)\n"
-      "True if the variable's value is empty, 0, FALSE, OFF, or NOTFOUND.\n"
+      "True if the variable's value is empty, 0, N, NO, OFF, FALSE, "
+      "NOTFOUND, or <variable>-NOTFOUND.\n"
       "  IF(variable1 AND variable2)\n"
       "True if both variables would be considered true individually.\n"
       "  IF(variable1 OR variable2)\n"