diff --git a/language/types.xml b/language/types.xml
index 5da7916527..d7fec8b5fd 100644
--- a/language/types.xml
+++ b/language/types.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.83 $ -->
+<!-- $Revision: 1.84 $ -->
  <chapter id="language.types">
   <title>Types</title>
 
@@ -1807,6 +1807,26 @@ $bar->do_foo();
       The null type was introduced in PHP 4
      </simpara>
     </note>
+    <para>
+     A variable is considered to be &null; if
+     <itemizedlist>
+      <listitem>
+       <para>
+        it has been assigned the constant &null;.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        it has not been set to any value yet.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        it has been <function>unset</function>.
+       </para>
+      </listitem>
+     </itemizedlist>
+    </para>
     
    <sect2 id="language.types.null.syntax">
     <title>Syntax</title>
@@ -1820,6 +1840,9 @@ $var = NULL;
       </programlisting>
      </informalexample>
     </para>
+    <para>
+     See also <function>is_null</function> and <function>unset</function>.
+    </para>
    </sect2>
   
   </sect1>
diff --git a/reference/var/functions/is-null.xml b/reference/var/functions/is-null.xml
index 7e290f57e4..224b8b223e 100644
--- a/reference/var/functions/is-null.xml
+++ b/reference/var/functions/is-null.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.2 $ -->
+<!-- $Revision: 1.3 $ -->
 <!-- splitted from ./en/functions/var.xml, last change in rev 1.35 -->
   <refentry id="function.is-null">
    <refnamediv>
@@ -19,7 +19,12 @@
      &false; otherwise.
     </para>
     <para>
-     See also <function>is_bool</function>,
+     See the <link linkend="language.types.null.syntax">&null;</link> type
+     when a variable is considered to be &null; and when not.
+    </para>
+    <para>
+     See also <link linkend="language.types.null.syntax">&null;</link>,
+     <function>is_bool</function>,
      <function>is_numeric</function>,
      <function>is_float</function>,
      <function>is_int</function>,