By the first quarter of the fourteenth century the methods of descant had become thoroughly obnoxious to the ecclesiastical authorities and the Pope, John XXII, issued a decree in 1322 for the restriction of descant and for the reëstablishing of plain-song. The old parallel organum of the fifth and fourth was still allowed. Singers, chafing under the severe restraint, added a third part between the cantus firmus and the fifth which on the written page looked innocent enough to escape detection, and further enriched the effect of their singing by transposing their plain-song to the octave above, which, as we have seen, then moved in the pleasant relation of the sixth to the written middle part. Thus, though the written parts looked in the book sufficiently like the old parallel organum, the effect of the singing was totally different. However, this explanation of the origin of the term _faux-bourdon_ leaves us still unenlightened as to how the sixth had come to sound so agreeably to the ears of these rebellious singers. Having perfected a system of notation, and having admitted the intervals pleasantest to our ears among the consonances to be allowed, having thus broadly widened their technique and the possibilities of music, we might well expect pleasing results from the mensuralists. But their music is, as a matter of fact, for the most part rigid and harsh. Several new forms of composition had been invented and had been perfected, notably by the two great organists of Notre Dame in Paris, Leo or Leonin, and his successor, Perotin. It is customary to group these compositions under three headings, namely, compositions in which all parts have the same words, compositions in which not all parts have words, and compositions in which the parts have different words. Among the first the _cantilena_ (_chanson_), the _rondel_ and _rota_ are best understood, though the distinction between the cantilena and the rondel is not evident. The rondel was a piece in which each voice sang a part of the same melody in turn, all singing together; but, whereas in the rota one voice began alone and the others entered each after the other with the same melody at stated intervals, until all were singing together, in the rondel all voices began together, each singing its own melody, which was, in turn, exchanged for that of the others. Among the compositions of the second class (in which not all parts have words), the _conductus_ and the _organum purum_ were most in favor. Both are but vaguely understood. The _organum purum_, evidently the survival of the old free descant, was written for two, three, or even four voices. The tenor sang the tones of a plain-song melody in very long notes, while the other voices sang florid melodies above it, merely to vocalizing syllables. The _conductus_ differed from this mainly in that such passages of florid descant over extended syllables of the plain-song were interspersed with passages in which the