material. This gave rise to the science of counterpoint, which, as I have said, consists of the trick of making a number of voices sing different melodies at the same time without violating certain given rules. The given melody or "principal" soon acquired the name of _cantus firmus_, and the other parts were each called _contrapunctus_,[11] as before they had been called tenor and descant. These names were first used by Gerson, Chancellor of Notre Dame, Paris, about 1400. In the meantime (about 1300-1375), the occasional use of thirds and sixths in the diaphonies previously explained led to an entirely different kind of singing, called _falso bordone_ or _faux bourdon_ (_bordonizare_, "to drone," comes from a kind of pedal in organum that first brought the third into use). This system, contrary to the old organum, consisted of using only thirds and sixths together, excluding the fourth and fifth entirely, except in the first and last bars. This innovation has been ascribed to the Flemish singers attached to the Papal Choir (about 1377), when Pope Gregory XI returned from Avignon to Rome. In the British Museum, however, there are manuscripts dating from the previous century, showing that the _faux bourdon_ had already commenced to make its way against the old systems of Hucbald and Guido. The combination of the _faux bourdon_ and the remnant of the organum gives us the foundation for our modern tone system. The old rules, making plagal motion of the different voices preferable to parallel motion, and contrary motion preferable to either, still hold good in our works on theory; so also in regard to the rules forbidding consecutive fifths and octaves, leaving the question of the fourth in doubt. To sum up, we may say, therefore, that up to the sixteenth century, all music was composed of the slender material of thirds, sixths, fifths, and octaves, fourths being permitted only _between_ the voices; consecutive successions of fourths, however, were permitted, a license not allowed in the use of fifths or octaves. This leads us directly to a consideration of the laws of counterpoint and fugue, laws that have remained practically unchanged up to the present, with the one difference that, instead of being restricted to the meagre material of the so-called consonants, the growing use of what were once called dissonant chords, such as the dominant seventh, ninth, diminished seventh, and latterly the so-called altered chords, has brought new riches to the art.