Standard Generalized Markup Language - Syntax

Syntax

An SGML document may have three parts:

1. the SGML Declaration,
2. the Prologue, containing a DOCTYPE declaration with the various markup declarations that together make a Document Type Definition (DTD), and
3. the instance itself, containing one top-most element and its contents.

An SGML document may be composed from many entities (discrete pieces of text). In SGML, the entities and element types used in the document may be specified with a DTD, the different character sets, features, delimiter sets, and keywords are specified in the SGML Declaration to create the concrete syntax of the document.

Although full SGML allows implicit markup and some other kinds of tags, the XML specification (s4.3.1) states:

Each XML document has both a logical and a physical structure. Physically, the document is composed of units called entities. An entity may refer to other entities to cause their inclusion in the document. A document begins in a "root" or document entity. Logically, the document is composed of declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup.

For introductory information on basic, modern SGML syntax, see XML. The following material concentrates on features not in XML and is not a comprehensive summary of SGML syntax.