diff --git a/man/man7/plumb.7 b/man/man7/plumb.7
index a3c10e1..82c771b 100644
--- a/man/man7/plumb.7
+++ b/man/man7/plumb.7
@@ -337,6 +337,11 @@
 The contents of the
 .B wdir
 field of the message.
+.TP
+.B $plan9
+The root directory of the Plan 9 tree
+(see
+.IR get9root (3)).
 .RE
 .SH EXAMPLE
 The following is a modest, representative file of plumbing rules.