diff --git a/man5/proc.5 b/man5/proc.5
index 92c49f93f..f9241b51f 100644
--- a/man5/proc.5
+++ b/man5/proc.5
@@ -1243,16 +1243,25 @@ mount ID: a unique ID for the mount (may be reused after
 .BR umount (2)).
 .TP
 (2)
-parent ID: the ID of the parent mount.
-For the root mount point,
-the ID shown here is a hidden mount ID associated with the mount namespace.
-That ID is distinct from any of the IDs shown in
-field (1) of the records shown in the
+parent ID: the ID of the parent mount
+(or of self for the root of this mount namespace's mount tree).
+.IP
+If the parent mount point lies outside the process's root directory (see
+.BR chroot (2)),
+the ID shown here won't have a corresponding record in
 .I mountinfo
-file, and does not appear in field (1) in the
-.I mountinfo
-file in any other mount namespace.
-(In the initial mount namespace, this hidden ID has the value 0.)
+whose mount ID (field 1) matches this parent mount ID
+(because mount points that lie outside the process's root directory
+are not shown in
+.IR mountinfo ).
+As a special case of this point,
+the process's root mount point may have a parent mount
+(for the initramfs filesystem) that lies
+.\" Miklos Szeredi, Nov 2017: The hidden one is the initramfs, I believe
+.\" mtk: In the initial mount namespace, this hidden ID has the value 0
+outside the process's root directory,
+and an entry for that mount point will not appear in
+.IR mountinfo .
 .TP
 (3)
 major:minor: the value of