Wikipedia: SGML

The Standard Generalized Markup Language (SGML) is a markup language for document?s. SGML is a standardized development from 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 single markup syntax that can be used for many applications. This prevents the need for multiple application-specific markup languages.

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:

 <QUOTE TYPE="example"> 
 typically something like <ITALICS>this</ITALICS> 
 </QUOTE>

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 small-scale 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.