The Standard Generalized Markup Language (SGML) is a metalanguage in which one can define markup languages for documents. SGML is a descendant of IBM's "Generalized Markup Language" GML, developed in the 1960s by Charles Goldfarb, Edward Mosher and Raymond Lorie (whose surname initials also happen to be GML).
SGML provides a variety of markup syntaxes that can be used for many applications. By changing the SGML Declaration one does not even need to use "angle brackets" although they are the norm.
SGML was originally designed to enable the sharing of machine-readable documents in large projects in government and the aerospace industry. It has also been used extensively in the printing and publishing industries.
SGML syntax example:
SGML is an ISO standard: "ISO 8879:1986 Information processing -- Text and office systems -- Standard Generalized Markup Language (SGML)"typically something like
SGML is very flexible and powerful, but its complexity has prevented its widespread application for small-scale general-purpose use.