diff --git a/cache.h b/cache.h
index b039abce0364736650aeec3d764c315aa3224008..9407560fcdcfb35740ef5b6fe551f731cbbabca4 100644
--- a/cache.h
+++ b/cache.h
@@ -798,13 +798,26 @@ static inline void *read_sha1_file(const unsigned char *sha1, enum object_type *
 {
 	return read_sha1_file_extended(sha1, type, size, LOOKUP_REPLACE_OBJECT);
 }
+
+/*
+ * This internal function is only declared here for the benefit of
+ * lookup_replace_object().  Please do not call it directly.
+ */
 extern const unsigned char *do_lookup_replace_object(const unsigned char *sha1);
+
+/*
+ * If object sha1 should be replaced, return the replacement object's
+ * name (replaced recursively, if necessary).  The return value is
+ * either sha1 or a pointer to a permanently-allocated value.  When
+ * object replacement is suppressed, always return sha1.
+ */
 static inline const unsigned char *lookup_replace_object(const unsigned char *sha1)
 {
 	if (!check_replace_refs)
 		return sha1;
 	return do_lookup_replace_object(sha1);
 }
+
 static inline const unsigned char *lookup_replace_object_extended(const unsigned char *sha1, unsigned flag)
 {
 	if (!(flag & LOOKUP_REPLACE_OBJECT))
diff --git a/replace_object.c b/replace_object.c
index c5cf9f4467c46e841a9fe02a42e7272f77de2556..31fabde72b5444cb965c72781ae451919ebc9797 100644
--- a/replace_object.c
+++ b/replace_object.c
@@ -92,6 +92,13 @@ static void prepare_replace_object(void)
 /* We allow "recursive" replacement. Only within reason, though */
 #define MAXREPLACEDEPTH 5
 
+/*
+ * If a replacement for object sha1 has been set up, return the
+ * replacement object's name (replaced recursively, if necessary).
+ * The return value is either sha1 or a pointer to a
+ * permanently-allocated value.  This function always respects replace
+ * references, regardless of the value of check_replace_refs.
+ */
 const unsigned char *do_lookup_replace_object(const unsigned char *sha1)
 {
 	int pos, depth = MAXREPLACEDEPTH;