DOCTYPE Declaration & DTDs

The document type (DOCTYPE) declaration consists of an internal, or references an external Document Type Definition (DTD). It can also have a combination of both internal and external DTDs. The DTD defines the constraints on the structure of an XML document. It declares all of the document's element types, children element types, and the order and number of each element type. It also declares any attributes, entities, notations, processing instructions, comments, and PE references in the document.