In Chapter 3, you learned about the physical and logical structure of an XML document, including the prolog that contains the XML declaration and the document type declaration. This chapter focuses on the prolog's document type declaration, which identifies the specific type of document being processed and the rules that govern the overall document. These rules are called the Document Type Definition, or DTD—by far the most involved part of the document type declaration.
Before getting into the details of creating a DTD, let's examine the document type declaration.