DTD

Document Type Definition

A DTD comprises rules for how documents of a certain type may or must look structurally speaking.The rules are written to describe a document type in a formal manner.

Since the XML standard has been developed to be extensible, unlike HTML it does not contain any fixed tags, but only rules regarding how tags need to look. If you want a certain type of document to be constructed with certain tags in a certain order, you can describe this in a document type definition or DTD.

In a DTD you can write rules that say that a document of the type "letter" must contain the building blocks "date", "subject", "description" and "sender" in that order. In recent years it has become increasingly common to describe the structure of an XML document by an "XML Schema" (*.xsd) instead, since this can be more advanced than a DTD.

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