Music Markup Language (MML) was an early application of XML to describe music objects and events.[1] MML pioneered features commonly used in later music markup formalisms, such as the IEEE 1599 standard. These features included the use of XML as a foundation; the ability to describe a musical object or event comprehensively (as opposed to merely providing a machine-readable format for a traditional musical score, or for a determinate sound recording of one performance); and the division of this comprehensive information into modules (often termed layers in later work), with separate modules for metadata, lyrics, notation, sound, and performance.[2][3][4][5] MML makes it possible to state relationships among written syllables, phonemes, notes in traditional musical notation, pitch, and rhythm in a flexible and extensible way.[6]: 222–223
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George, Susan Ella (2005). "Chapter 7: Lyric recognition and Christian music". In George, Susan Ella (ed.). Visual perception of music notation: on-line and off-line recognition. Hershey, Pennsylvania: IRM Press. ISBN1-59140-298-0.