SGML (Standard Generalized Markup Language) is the predecessor to XML. Like XML, it is a generic way to define a mark-up language or tag set, thus allowing applications to create custom tag-based languages for specific purposes. HTML is one such language. Many people thought that SGML was too general and too complicated for most needs. Thus, XML, a more restrictive version of SGML, has quickly surpassed SGML in popularity. XHTML is the new version of HTML based on the simpler XML.