diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 45a865c..e8ef990 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -171,7 +171,9 @@
 static char math_frexp_doc [] =
 "frexp(x)\n\
 \n\
-Return the matissa and exponent for x. The mantissa is positive.";
+Return the matissa and exponent of x, as pair (m, e).\n\
+m is a float and e is an int, such that x = m * 2.**e.\n\
+If x is 0, m and e are both 0.  Else 0.5 <= abs(m) < 1.0.";
 
 
 static PyObject *