Consider the following DTD snippet....

	<!ELEMENT CONTACT (NAME, EMAIL)>
	<!ELEMENT NAME (#PCDATA)>
	<!ELEMENT EMAIL (#PCDATA)>

In this case, we expect to see XML along the lines of

        <CONTACT>
        <NAME>Jim Sanger</NAME>
        <EMAIL>sanger@sanger.com</EMAIL>
        </CONTACT>

Alternatively, the following code would be valid:

        <CONTACT>
        <EMAIL>sanger@sanger.com</EMAIL>
        <NAME>Jim Sanger</NAME>
        </CONTACT>

We used a comma to order the list because all children must be ordered. We could use a pipe to delimit a list of non-ordered, optional elements, however. [thanks to Jason Suwala for pointing our error on unordered children--ed]. Thus if we redefined our DTD to use

	<!ELEMENT CONTACT (NAME, EMAIL)>
	<!ELEMENT NAME (#PCDATA)>
	<!ELEMENT EMAIL (#PCDATA)>

Then the following XML would be valid

        <CONTACT>
        <NAME>Jim Sanger</NAME>
        <EMAIL>sanger@sanger.com</EMAIL>
        </CONTACT>

but the following XML would be invalid because the EMAIL element would not be allowed to precede the NAME element.

        <CONTACT>
        <EMAIL>sanger@sanger.com</EMAIL>
        <NAME>Jim Sanger</NAME>
        </CONTACT>