Magnus Liber Organi

(Lat. "Great Book of Organum")

   Collection of organum associated with the cathedral of Notre Dame in Paris and believed to have been composed between 1160 and 1225. No manuscript with the title exists; the collection has been reconstructed on the basis of later collections containing the repertory described by a 13th-century English student known as Anonymous IV, who cites Leoninus and Perotinus as the principal composers. This reconstruction contains 35 responsories and 12 settings of the Benedicamus Domino for the divine office, and 20 Graduals and 40 Alleluias for the mass. There are also 500 clausulae that may be substituted for portions of the melismatic organum with the same text. In most cases, the polyphony is intended to be sung in alternation with traditional chant, which completes a respond or supplies a verse.

   See also Alternatim.