This International Standard specifies an abstract syntax known as the Standard Generalized Markup Language (SGML). The language expresses the description of a document's structure and other attributes, as well as other information that makes the markup interpretable.
ANSI/ISO 8879:1986 history
1986ANSI/ISO 8879:1986 Information processing - Text and office systems - Standard Generalized Markup Language (SGML) Adopted by INCITS