The Micrologus is a treatise on Medieval music written by Guido of Arezzo, dating to 1050 A.D.. This treatise outlines singing and teaching practice for Gregorian chant, and has considerable discussion of the composition of polyphonic music. This treatise discusses modified parallel organum as well as free organum. The examples given are in two voices, set note-against-note, and the voices are frequently permitted to cross. He advised against use of the perfect fifth and minor second, favouring… (More on Micrologus)