diff --git a/du.1 b/du.1
index 211d83f..262d314 100644
--- a/du.1
+++ b/du.1
@@ -47,3 +47,11 @@ Don't dereference symbolic links. This is the default.
 .It Fl x
 Do not traverse file systems mount points.
 .El
+.Sh STANDARDS
+The
+.Nm
+utility is compliant with the
+.St -p1003.1-2008
+specification. The
+.Op Fl P
+flag is an extension to that specification.