diff --git a/include/openssl/ec.h b/include/openssl/ec.h
index b34605f..fd35b03 100644
--- a/include/openssl/ec.h
+++ b/include/openssl/ec.h
@@ -209,6 +209,9 @@
 // EC_POINT_get_affine_coordinates_GFp sets |x| and |y| to the affine value of
 // |point| using |ctx|, if it's not NULL. It returns one on success and zero
 // otherwise.
+//
+// Either |x| or |y| may be NULL to skip computing that coordinate. This is
+// slightly faster in the common case where only the x-coordinate is needed.
 OPENSSL_EXPORT int EC_POINT_get_affine_coordinates_GFp(const EC_GROUP *group,
                                                        const EC_POINT *point,
                                                        BIGNUM *x, BIGNUM *y,