The Standardized Generalized Markup Language (SGML) is a markup language standardized by the International Standards Organization( ISO) for describing electronic documents in a uniform manner. It does not describe the document content, but the hierarchy and the individual text components such as author, heading, commands and code. This allows documents to be exchanged regardless of their file format.
Data files written in SGML consist of ASCII characters and have no formatting characters; they contain descriptive codes. The beginning and end of logical document elements are marked by an SGML character. These SGML markers are information objects that also refer to other files. How the document looks later in the layout is determined by the Document Type Definition( DTD) and the output medium. The tags of SGML are flexibly interpretable, only the mechanisms of the tags are described.