diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index fcd88ad..c313f35 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -106,7 +106,8 @@
 FUNC1(floor, floor,
       "floor(x)\n\nReturn the floor of x as a real.")
      FUNC2(fmod, fmod,
-      "fmod(x,y)\n\nReturn x % y.")
+      "fmod(x,y)\n\nReturn fmod(x, y), according to platform C."
+      "  x % y may differ.")
 FUNC2(hypot, hypot,
       "hypot(x,y)\n\nReturn the Euclidean distance, sqrt(x*x + y*y).")
 FUNC1(log, log,