XML is a system-neutral Internet format for representing structured data. It is a subset of Standard Generalized Markup Language (SGML), which was developed in the Seventies for the interchange of text files between printers. The primary motivation behind XML is to simplify and improve delivery of information via the Internet. The first version of XML was designed by the World Wide Web Consortium (W3C) in 1998.
XML has several secondary objectives. It defines how Internet Uniform Resource Locators [URLs] can be used to identify component parts of XML data streams. It removes inconsistencies and ambiguities of earlier markup languages. Unlike SGML and HyperText Markup Language (HTML), it forces users to adhere to strict data-models and provides syntactical error-checking.
In contrast to HTML, XML emphasizes elements of content, as opposed to typographical style, layout, or form. Like SGML and HTML, XML involves extensive tagging, but the focus of the tagging is to identify logical types of information (in music such types might include beams, bars, and rests). Although default tags are available in XML, they are often impractical for document exchange on the Internet.
Of particular benefit to music applications is the fact that XML allows support for multiple descriptions of the same kinds of data on a continuum of “completeness.” The task of formatting documents is relegated to other software adapted to read XML data descriptions (“schemas”), formally called Document Type Definitions (DTDs).