DTD-DOCUMENT TYPE DEFINITION

Document Type Definition (DTD) is one of several SGML and XML schema languages, and is also the term used to describe a document or portion thereof that is authored in the DTD language. A DTD is primarily used for the expression of a schema via a set of declarations that conform to a particular markup syntax and that describe a class, or type, of document, in terms of constraints on the structure of that document. A DTD may also declare constructs that are not always required to establish document structure, but that may affect the interpretation of some documents. XML documents are described using a subset of DTD which imposes a number of restrictions on the document's structure, as required per the XML standard (XML is in itself an application of SGML optimized for automated parsing). DTDs are written in a formal syntax that explains precisely which elements and entities may appear where in the document and what the elements’ contents and attributes are. DTD is native to the SGML and XML specifications, and since its introduction other specification languages such as XML Schema and RELAX NG have been released with additional functionality.