diff --git a/docs/xml/manual-writing-tools.xml b/docs/xml/manual-writing-tools.xml
index b491f9880..55da11522 100644
--- a/docs/xml/manual-writing-tools.xml
+++ b/docs/xml/manual-writing-tools.xml
@@ -123,7 +123,8 @@ top-level directory <computeroutput>valgrind/</computeroutput>.</para>
   <listitem>
    <para>Edit <filename>Makefile.am</filename>, adding the new directory
    <filename>foobar</filename> to the
-   <computeroutput>TOOLS</computeroutput> variable.</para>
+   <computeroutput>TOOLS</computeroutput> and
+   <computeroutput>SUBDIRS</computeroutput>variables.</para>
   </listitem>
 
   <listitem>