ABSTRACT
Document Type Definitions (DTDs) are schemas that describe the structure and, to a limited extent, the content of Extensible Markup Language (XML) and Standard Generalized Markup Language (SGML) documents. At its inception, the XML standard inherited the DTD from SGML as its only schema language. Many alternative schema languages have subsequently been developed for XML. But the DTD is still alive and actively used to define narrative-based document types. This entry describes the basic syntax of the DTD and compares it to its two main rivals: W3C XML Schema and RELAX NG.
