diff --git a/hash.h b/hash.h
index 0d3d85e04c..953e840d15 100644
--- a/hash.h
+++ b/hash.h
@@ -176,6 +176,8 @@ static inline void git_SHA256_Clone(git_SHA256_CTX *dst, const git_SHA256_CTX *s
 #define GIT_HASH_NALGOS (GIT_HASH_SHA256 + 1)
 /* Default hash algorithm if unspecified. */
 #define GIT_HASH_DEFAULT GIT_HASH_SHA1
+/* Legacy hash algorithm. Implied for older data formats which don't specify. */
+#define GIT_HASH_SHA1_LEGACY GIT_HASH_SHA1
 
 /* "sha1", big-endian */
 #define GIT_SHA1_FORMAT_ID 0x73686131