Advance XHTML Programming Question:

SGML (Standard Generalized Markup Language) is a metalanguage in which one can define markup languages, like HTML, 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).

XHTML is not directly related to SGML. But it is indirectly to SGML through HTML.