diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 7d587b226c..1afd272ff0 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -936,6 +936,30 @@ CREATE TABLE t1 (
     in the usual way.
    </para>
 
+   <indexterm>
+    <primary>foreign key</primary>
+    <secondary>self-referential</secondary>
+   </indexterm>
+
+   <para>
+    Sometimes it is useful for the <quote>other table</quote> of a
+    foreign key constraint to be the same table; this is called
+    a <firstterm>self-referential</firstterm> foreign key.  For
+    example, if you want rows of a table to represent nodes of a tree
+    structure, you could write
+<programlisting>
+CREATE TABLE tree (
+    node_id integer PRIMARY KEY,
+    parent_id integer REFERENCES tree,
+    name text,
+    ...
+);
+</programlisting>
+    A top-level node would have NULL <structfield>parent_id</structfield>,
+    but non-NULL <structfield>parent_id</structfield> entries would be
+    constrained to reference valid rows of the table.
+   </para>
+
    <para>
     A table can have more than one foreign key constraint.  This is
     used to implement many-to-many relationships between tables.  Say