diff --git a/man7/cgroups.7 b/man7/cgroups.7
index aa3127a05..35f8bd634 100644
--- a/man7/cgroups.7
+++ b/man7/cgroups.7
@@ -1328,9 +1328,30 @@ to the
 files in different cgroups inside the subtree.
 The threads of a process must all reside in the same threaded subtree.
 .PP
-The
+As with writing to
+.IR cgroup.procs ,
+some containment rules apply when writing to the
 .I cgroup.threads
-file is writable only for the cgroups inside a threaded subtree.
+file:
+.IP * 3
+The writer must have write permission on the
+cgroup.threads
+file in the destination cgroup.
+.IP *
+The writer must have write permission on the
+.I cgroup.procs
+file in the common ancestor of the source and destination cgroups.
+(In some cases,
+the common ancestor may be the source or destination cgroup itself.)
+.IP *
+The source and destination cgroups must be in the same threaded subtree.
+(Outside a threaded subtree, an attempt to move a thread by writing
+its thread ID to the
+.I cgroup.threads
+file in a different
+.I domain
+cgroup fails with the error
+.BR EOPNOTSUPP .)
 .PP
 The
 .I cgroup.threads