Before going on, a quick note about designing XML document types for flexibility. Consider the sample name and address document type; I clearly wrote it with U.S. postal addresses in mind. If you want a DTD or schema that defines rules for other types of addresses, you would have to add a lot more complexity to it. Requiring a <state> element might make sense in Australia, but it wouldn't in the UK. A Canadian address might be handled by the sample DTD in Document Type Definitions, but adding a <province> element is a better idea. Finally, be aware that in many parts of the world, concepts like title, first name, and last name don't make sense.

The bottom line: If you're going to define the structure of an XML document, you should put as much forethought into your DTD or schema as you would if you were designing a database schema or a data structure in an application. The more future requirements you can foresee, the easier and cheaper it will be for you to implement them later.