About

Document Type Definition (DTD) is a grammar (language) that defines the schema (ie data structure) of an XML document.

It

• defines constraints on the logical structure
• supports the use of predefined storage units (physical structure).

Artuicles Related

Syntax

The syntax is a set of markup declarations that define a document type for SGML-family markup languages (SGML, XML, HTML).

DTDs were a precursor to XML schemas and have a similar function, although different capabilities.

• DTD’s describe XML tags with a Regular expression format like person (name, addr, child*).
• DTD’s taken as a whole, are context-free grammar (CFG).

A DTD can be declared:

• inline inside an XML document,
• or as an external reference.

The document type declaration can point to an external subset (a special kind of external entity) containing:

• markup declarations,
• or can contain the markup declarations directly in an internal subset,
• or can do both.

The DTD for a document consists of both subsets taken together.

DTD are context-free grammars

Documentation / Reference

• wiki/Document Type Definition
• Prolog and Document Type Declaration