diff --git a/Makefile b/Makefile
index 0c79546712..7d0fa7adb6 100644
--- a/Makefile
+++ b/Makefile
@@ -500,6 +500,11 @@ include shared.mak
 # Define BLK_SHA1 to make use of optimized C SHA-1 routines bundled
 # with git (in the block-sha1/ directory).
 #
+# Define NO_APPLE_COMMON_CRYPTO on OSX to opt-out of using the
+# "APPLE_COMMON_CRYPTO" backend for SHA-1, which is currently the
+# default on that OS. On macOS 01.4 (Tiger) or older,
+# NO_APPLE_COMMON_CRYPTO is defined by default.
+#
 # If don't enable any of the *_SHA1 settings in this section, Git will
 # default to its built-in sha1collisiondetection library, which is a
 # collision-detecting sha1 This is slower, but may detect attempted