Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

SGML is an open standard markup language that specifies how documents should be formatted. In this sense, it is a metalanguage-a language that describes a formatting and markup language. The term markup is historically based on the marks made by copy editors to pages that indicate how they should be formatted and typeset. In the early days of computer typesetting, there were many different typesetting systems and each used its own proprietary markup language. This language consisted of special control characters to indicate the beginning and end of some formatting. The markups were so obscure that users quickly realized a standard markup language was needed to reduce confusion.

Two organizations, the Graphics Communications Association and ANSI, went to work on the problem in 1980. They eventually combined their work, and in 1986, the ISO introduced it as SGML (Standard Generalized Markup Language). The most important part of SGML is that files contain standard ASCII text, which means they are portable from one system to another. SGML goes beyond simple document formatting by defining document structures and relationships. Document parts are defined in a hierarchical tree, and formatting is applied based on that hierarchy. Information in documents is translated to perform actions or formatting on other systems.

An SGML document consists of the actual text file (called the document instance) and a separate DTD (Document Type Definition) file. The DTD specifies the rules for tagging the document and defines all the ways that the document instance can be laid out and formatted. Since the DTD defines how the associated document will look, the DTD can be changed at any time to alter the document's appearance. In addition, DTDs can be created by various industries and organizations to define specific types of document layouts.

HTML, the markup language used for Web pages, is related to SGML. More recently, XML (Extensible Markup Language) was introduced as a way to define the elements in documents with tags so they can be extracted into databases, spreadsheets, and by programs.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.