diff --git a/reference/image/functions/imagettfbbox.xml b/reference/image/functions/imagettfbbox.xml
index 1296c117ab..a1bde92512 100644
--- a/reference/image/functions/imagettfbbox.xml
+++ b/reference/image/functions/imagettfbbox.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.11 $ -->
+<!-- $Revision: 1.12 $ -->
 <refentry xml:id="function.imagettfbbox" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
  <refnamediv>
   <refname>imagettfbbox</refname>
@@ -27,7 +27,8 @@
      <term><parameter>size</parameter></term>
      <listitem>
       <para>
-       The font size in pixels.
+       The font size. Depending on your version of GD, this should be
+       specified as the pixel size (GD1) or point size (GD2).
       </para>
      </listitem>
     </varlistentry>