Standard Generalized Markup Language

Standard Generalized Markup Language
Name	Standard Generalized Markup Language
Released	October 1986
Status	International Standard (ISO 8879:1986)
Genre	Markup language
Influenced by	GML (IBM), Scribe (markup language)
Influenced	HTML, XML, DocBook, TEI

Standard Generalized Markup Language. It is an international standard (ISO 8879) for defining generalized markup languages for documents. Developed in the early 1980s, it provides a framework for describing the structure and content of a document independently of its presentation. Its primary goal is to enable document portability across different hardware and software systems, ensuring long-term preservation and reuse of information.

History and development

The origins of the language can be traced to work done at IBM in the late 1960s, notably Charles Goldfarb, Edward Mosher, and Raymond Lorie, who created the GML (IBM) system. This early work evolved through the efforts of the Graphic Communications Association and was heavily influenced by the design of Scribe (markup language) developed by Brian Reid (computer scientist). The formal standardization process began under the auspices of the International Organization for Standardization (ISO), with Goldfarb acting as the principal editor. After several years of committee work within ISO/IEC JTC 1, the standard was officially published in October 1986. Key contributions to its theoretical underpinnings also came from researchers involved with the Association for Computing Machinery and the American National Standards Institute.

Design principles

The design is fundamentally based on the separation of document structure from processing and presentation. A core tenet is the use of descriptive, rather than procedural, markup, where tags describe what an element is, not how it should look. It introduced the revolutionary concept of a formal Document Type Definition (DTD) to define the legal building blocks of a document class. The language is also designed to be system-independent, facilitating the exchange of documents between disparate platforms like those from Digital Equipment Corporation and Apple Inc.. Furthermore, it supports the international character set standards of the International Organization for Standardization, promoting global use.

Components and syntax

The syntax revolves around elements delimited by start-tags and end-tags, such as ``. These elements can contain parsed character data, other nested elements, or a mixture of both. Key syntactic constructs include SGML entity references, which act as placeholders for text or special characters, and SGML declarations, which define the concrete syntax for a given application. The handling of attributes within start-tags allows for additional information about an element. Processing instructions, comments, and marked sections provide mechanisms for embedding application-specific commands or conditional text within the document instance.

Document Type Definition (DTD)

The Document Type Definition is a formal grammar that defines the structure and constraints for a class of documents. It declares the allowable SGML element types, their permissible attributes, and the hierarchical context in which they can appear. The DTD also declares entities, including external entities that can incorporate content from other files or data streams. This separation of schema (the DTD) from instance (the actual document) was a foundational idea later adopted by XML. Validating a document against its DTD ensures conformance to the structural rules, a process crucial for large-scale publishing systems used by organizations like the United States Department of Defense and the Internal Revenue Service.

Applications and influence

It found early and significant adoption in industries requiring complex, long-lived documentation. Major applications included technical documentation in aerospace and defense sectors, exemplified by the United States Air Force and its integration with the Integrated Computer-Aided Manufacturing project. It was the basis for large publishing systems at institutions like Oxford University Press and the Library of Congress. Its most profound influence was as the direct ancestor of HTML, the language of the World Wide Web, and the more rigorous XML, which underpins countless modern data formats. Other notable derived languages include DocBook for technical documentation and the Text Encoding Initiative (TEI) guidelines for humanities scholarship.

Relationship to other markup languages

It is the meta-language from which many other languages are derived. HTML, as defined in the early specifications by the World Wide Web Consortium (W3C), was an application of it before evolving independently. XML, developed by the World Wide Web Consortium in the late 1990s, is a streamlined, simplified subset designed for easier parsing and web interoperability. While XML retains the core concepts of elements, attributes, and DTDs, it removes many of the more complex and optional features. Other languages like HyperCard used different paradigms, but the widespread adoption of its principles in XML solidified its architectural influence on data representation across industries from Silicon Valley to European Union institutions.

Category:Markup languages Category:ISO standards Category:Document markup languages