Magnus Liber

The Magnus Liber or Magnus Liber Organi (Latin for "Great Book of Organum") is a compilation of the medieval music known as organum. The whole name of the work is Magnus liber organi de graduali et antiphonario pro servitio divino. Written during the 12th and early 13th centuries, this series of compositions is attributed to masters of the Notre Dame school of music, most notably Léonin and his successor Pérotin. (These names survive today because of the testimony of an English music theorist known simply as Anonymous IV.)

The Magnus Liber represents a step in the evolution of Western music between plainchant and the intricate polyphony of the later 13th and 14th centuries (see Machaut and Ars Nova). The music of the Magnus Liber displays a connection to the emerging Gothic style of architecture; just as ornate cathedrals were built to house holy relics, organa were written to elaborate Gregorian chant, which too was considered holy. One voice sang the notes of the Gregorian chant elongated to enormous length (called the tenor, which comes from the Latin for "to hold"); this voice, known as the vox principalis, held the chant, although the words were obscured by the length of notes. One, two, or three voices, known as the vox organalis (or vinnola vox, the "vining voice") were notated above it with quicker lines moving and weaving together. The evolution from a single line of music to one where multiple lines all had the same weight moved through the writing of organa. The practice of keeping a slow moving "tenor" line continued into secular music, and the words of the original chant survived in some cases, as well. One of the most common types of organa in the Magnus Liber is the clausula, which are sections of polyphony that can be substituted into longer organa.

The music of the Magnus Liber was used in the liturgy of the church throughout the feasts of the church year. The text contains only the polyphonic lines and the notation is not exact, as barlines were still several centuries from invention. The chant was added to the notated music, and it was up to the performers to fit the disparate lines together into a coherent whole. But the fact that the music was even written down is a fairly new development in the history of Western music.