A DTD allows you to specify the basic structure of an XML document. The next couple of panels look at fragments of DTDs. First of all, here's a DTD that defines the basic structure of the address document example in the section, What is XML?:

<!-- address.dtd -->
<!ELEMENT address (name, street, city, state, postal-code)>
<!ELEMENT name (title? first-name, last-name)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT first-name (#PCDATA)>
<!ELEMENT last-name (#PCDATA)>
<!ELEMENT street (#PCDATA)>
<!ELEMENT city (#PCDATA)>
<!ELEMENT state (#PCDATA)>
<!ELEMENT postal-code (#PCDATA)>
          

This DTD defines all of the elements used in the sample document. It defines three basic things:

• An <address> element contains a <name>, a <street>, a <city>, a <state>, and a <postal-code>. All of those elements must appear, and they must appear in that order.
• A <name> element contains an optional <title> element (the question mark means the title is optional), followed by a <first-name> and a <last-name> element.
• All of the other elements contain text. (#PCDATA stands for parsed character data; you can't include another element in these elements.)

Although the DTD is pretty simple, it makes it clear what combinations of elements are legal. An address document that has a <postal-code> element before the <state> element isn't legal, and neither is one that has no <last-name> element.

Also, notice that DTD syntax is different from ordinary XML syntax. (XML Schema documents, by contrast, are themselves XML, which has some interesting consequences.) Despite the different syntax for DTDs, you can still put an ordinary comment in the DTD itself.