SGML - Text markup language

SGML, aka Standard Generalized Markup Language, is a text markup language created in 1986.

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The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized markup languages for documents. ISO 8879 Annex A.1 defines generalized markup: Generalized markup is based on two postulates: Markup should be declarative: it should describe a document's structure and other attributes, rather than specify the processing to be performed on it. Declarative markup is less likely to conflict with unforeseen future processing needs and techniques. Read more on Wikipedia...

SGML Wikipedia page
SGML first developed in IBM
SGML on HOPL
See also: (14 related languages)GML, HTML, XML, Scheme, Linux, HyTime, Regular Expressions, Unicode, XQuery, JSP, Scala, DTD, S-expressions, LaTeX

Example from Wikipedia:

<lines>
<line>first line</line>
<line>second line</line>
</lines>