novembre 26th, 2012 9 h 26 min

Because church modes still prevails in numerous cultures, « Desertion » could be a proper word.

Actually the reducing evolution of mode that give birth to tonal music,based upon only two modes, is a very slow process that makes difficult to find a proper word to describe it.

There are many reasons that lead to the disappearance of modal scales.

    • One of the main reasons results from the way to avoid the triton F-B in polyphonies by lowering B, so it becomes Bb, or altering F into F#.
    • the second reason is a rule of counterpoint: an imperfect consonance(thirds and sixths) followed by a perfect  consonance is better approached by a semi-tone in one part and one tone in the other part
      A or B are better than C

      In three part this rule yields the double leading tones cadence which is of two kinds

      1-Machaut cadence:The two upper parts contain an half tone, opposed to the Bass:

      Adding a cambiata produces a Landini cadence

      2-Phrygian cadence:The two upper parts contain a tone, opposed to the bass

The third factor is the movement to a fifth lower at the cadence which created a forbidden dissonance seventh with the double leading tones cadence. So the authentic cadence is born: a major third resolving into an octave and a movement of fifth but leading to the disappearance of double leading tones cadence the F mode and G mode at the advantage of the C mode due to the necessary adaptation to avoid the dissonance seventh

To summarize we can say the main cause of the change is the growth of polyphony

* X th century with melody accompanied with parallel fifths or fourths.
* XI th century the melody starts and ends with the same note (named final, now tonic) and is accompanied with oblique motion which begins and ends in unison preceded by major second.
* XI-XIIth century introduction of the contrary motion with a major second as pre-cadential dissonance and a fifth added to the tonic
* The third is henceforth a imperfect consonance (formerly a dissonance) XIII th century cadence with double leading tone according to the rule of counterpoint edicted by Jean De Murs) The final chord has no third.
Consonance (8-5-4) evenly appear at the beginning of each « perfection » (group of 3 breves); there are non harmonic tones in short values between groups
* Ars Nova give up the rhythmic modes so consonances may appear on weak beat (XIVth century); the third is included into the fifth chord except in final position. Appearance of the movement V-I at the bass and the movement VII-I at the soprano {G-D-G}-{CGC}
* XVth Century: Instrumental music spread over the sonorous space with non modulating sequences on each degree. The third appears on the final chord so the authentic cadence is completed
* XVI-XVIIth centuries is period of great change:alteration to approach consonance by half step resulting in 12 divisions of the octave and the polyphonies that jut out the octave impeded to distinguish the authentic and plagal forms-another cause of disappearance of modes Adoption of the Zarlino’s system with pure third and false fifths will yield new rules of counterpoint such as to forbid direct fifth and change the rule of third and sixth which become perfect consonance.

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