diff --git a/include/fil0fil.h b/include/fil0fil.h
index 53bb496c190..6b8fd4b03d5 100644
--- a/include/fil0fil.h
+++ b/include/fil0fil.h
@@ -57,9 +57,21 @@ extern fil_addr_t	fil_addr_null;
 					page */
 #define FIL_PAGE_OFFSET		4	/* page offset inside space */
 #define FIL_PAGE_PREV		8	/* if there is a 'natural' predecessor
-					of the page, its offset */
+					of the page, its offset.
+					Otherwise FIL_NULL.
+					This field is not set on BLOB pages,
+					which are stored as a singly-linked
+					list.  See also FIL_PAGE_NEXT. */
 #define FIL_PAGE_NEXT		12	/* if there is a 'natural' successor
-					of the page, its offset */
+					of the page, its offset.
+					Otherwise FIL_NULL.
+					B-tree index pages
+					(FIL_PAGE_TYPE contains FIL_PAGE_INDEX)
+					on the same PAGE_LEVEL are maintained
+					as a doubly linked list via
+					FIL_PAGE_PREV and FIL_PAGE_NEXT
+					in the collation order of the
+					smallest user record on each page. */
 #define FIL_PAGE_LSN		16	/* lsn of the end of the newest
 					modification log record to the page */
 #define	FIL_PAGE_TYPE		24	/* file page type: FIL_PAGE_INDEX,...,
diff --git a/include/rem0rec.h b/include/rem0rec.h
index b2f3b9d141b..af11198d001 100644
--- a/include/rem0rec.h
+++ b/include/rem0rec.h
@@ -19,8 +19,10 @@ Created 5/30/1994 Heikki Tuuri
 #define REC_MAX_HEAP_NO		(2 * 8192 - 1)
 #define REC_MAX_N_OWNED		(16 - 1)
 
-/* Flag denoting the predefined minimum record: this bit is ORed in the 4
-info bits of a record */
+/* Info bit denoting the predefined minimum record: this bit is set
+if and only if the record is the first user record on a non-leaf
+B-tree page that is the leftmost page on its level
+(PAGE_LEVEL is nonzero and FIL_PAGE_PREV is FIL_NULL). */
 #define REC_INFO_MIN_REC_FLAG	0x10UL
 
 /* Number of extra bytes in an old-style record,