As we have already alluded to several times throughout this tutorial, a DTD can either be included as part of a well-formed XML document (standalone=yes) or it can be referenced from an external source (standalone=no).

The benefits of using external DTDs is that they can more easily and efficiently be shared by more than one XML document, or in fact, many organizations with the need to standardize communications and data. You can write a DTD once and have multiple documents reference it. Not only does this save typing time, but it assures that as the DTD manager makes changes to the central DTD, all documents that rely on the DTD are updated in one fell swoop (Of course, DTD changes will not necessarily be backwards compatible, so watch out!).

In order to reference an external DTD, you must change both the XML declaration and the DOCTYPE declaration.

The XML Declaration must be changed to reflect the fact that the XML document will not work on its own. That is, it will not be standalone.

        <?xml version = "1.0"
	             encoding="UTF-8"
	             standalone = "no"?>

You will also need to change the DOCTYPE declaration to add the SYSTEM attribute.

        <!DOCTYPE ROOT_ELEMENT
	             SYSTEM "URL_OF_EXTERNAL_DTD">

such as....

        <!DOCTYPE CONTACTS
	             SYSTEM "http://www.mydomain.com/dtds/contacts.dtd">

Note also, that the URL may be a relative or absolute file location such as....

        <!DOCTYPE CONTACTS
	             SYSTEM "contacts.dtd">

which specifies a dtd file in the same directory as the XML document that references it. Or, similarly, you can reference the same document up one directory and down one into the "dtds" directory.

        <!DOCTYPE CONTACTS
	             SYSTEM "../dtds/contacts.dtd">

Using this method, you can simply cut out the DTD from your XML document and paste it into the separate document called contacts.dtd. Thus, you have one file with the DTD and one file with the well-formed XML document.