SGML

SGML, Standard Generalized Markup Language, is a standard meta-language for defining markup languages. It enables authors to specify the structure and tagging rules for documents, allowing customizable markup schemes across different domains.

Developed in the 1980s and published by the International Organization for Standardization as ISO 8879:1986, SGML provides a formal framework for describing document structure. It is widely associated with publishing, government documentation, and standards a societal use, where large collections of complex documents must be managed consistently.

Core concepts of SGML include the Document Type Definition (DTD), which declares the allowed elements and their relationships, and the content models that specify how elements may be arranged (such as mixed or element-only content). It also uses attributes, default values, and entity declarations (including unparsed entities and notations) to control data representation. Parsing and validation are integral, with conforming SGML parsers checking a document against its DTD.

SGML has had a lasting influence on later markup languages. HTML, the foundational language of the Web, derives from SGML concepts, and XML was designed as a more compact, simpler subset of SGML by the W3C in the 1990s. While XML has become the dominant choice for new markup schemes, SGML remains in use in certain industries and for large, long-lived document workflows, such as in publishing and standards bodies, where its flexibility and formalism continue to be valued.