Standard Generalized Markup Language
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The Standard Generalized Markup Language (SGML) is a markup language for documents.
SGML was originally designed to enable the sharing of machine-readable documents in large projects. It has also been used extensively in the printing and publishing industries.
SGML is an ISO standard: "ISO 8879:1986 Information processing -- Text and office systems -- Standard Generalized Markup Language (SGML)"
SGML is very flexible and powerful, but its complexity has prevented its widespread application for general-purpose use. XML is an attempt to simplify SGML for general-purpose applications, such as the semantic web.
HTML and XML can both be regarded as derived from, and applications of, SGML.