Remain in the mode, except for conventional departures.
The ecclesiastical modes found in monophonic chant (Dorian, Phrygian, etc.) assumed certain melodic alterations of modal tones, and the nature of the modes changed further when they began to be used in polyphony. In his Counterpoint Knud Jeppeson identifies several general principles for the introduction of tones outside the mode:
(1) With the exception of the Phrygian, it became customary in cadences to the tonic to raise the seventh degree if the mode lacked a semitone below the tonic. In this way most of the modes came to resemble our modern major and minor scales.
(2) In descending, the upper note of the mode's tritone (B when untransposed) was often flatted to avoid the tritone if the lower note (F when untransposed) was present. In ascending, the same problem was solved by raising the lower pitch.
(3) Upper auxiliary tones (upper neighbor tones) were typically flatted in practice, though generally not written that way.
(4) The sixth degree in a minor mode could be raised if moving to or from a raised seventh degree, in order to avoid a melodic augmented second. This is the basis of the modern "melodic minor." In certain cases it might be necessary to raise another degree to avoid the aug. 2nd, for example in a Phrygian final cadence where the third is raised to major and a voice passes from the second degree to the raised third degree.
(5) Internal cadences could be formed on any degree, with one tone raised to form a leading-tone. But the modes had characteristic differences in the tones on which such cadences were formed. Jeppeson identifies these patterns, with the three most common cadences as follows:
Dorian: cadences on tonic, dominant, mediant
Phrygian: cadences on tonic, subdominant, mediant
Lydian: Because the fourth degree is typically flatted, it's essentially like transposed Ionian.
Mixolydian: cadences on tonic, dominant, subdominant
Aeolian: cadences on tonic, subdominant, mediant
Ionian: cadences on tonic, dominant, submediant
Countepointer will allow modal alterations needed to provide leading tones to any of these most common cadence points.
In polyphonic music there is no distinction between plagal and authentic modes (e.g. HypoDorian and Dorian), so these six are all that are required.
This complex set of conditions for the introduction of nonmodal tones is represented in Counterpointer by a single rule found in the Harmony style window:
Remain in the mode, except for conventional departures.
When this rule is in force you will be expected to keep to the notes of the mode of the cantus firmus, with these exceptions and conditions:
Mode pitches can be raised to form an internal cadence to a mode-appropriate
cadence tone as described above. But a raised leading tone must be approached by step or by a descending leap. The resolution
to the cadence tone is most often immediate, but these extended resolutions will be recognized:
The upper tone of the mode's tritone can
be lowered to avoid emphasizing the tritone, or, especially in ascending motion the lower tone can be raised.
In untransposed modes (all Counterpointer species exercises unless modified) the two notes
involved are F and B. In general terms for transposed modes these are degrees 3-6 in Dorian, 2-5 in Phrygian,
1-4 in Lydian, 7-3 in Mixolydian, 6-2 in Aeolian, and 4-7 in Ionian.
The upper auxiliary is typically lowered if it is not already a halfstep.
The sixth degree may be raised to avoid a melodic augmented second with
a raised seventh. In general a degree can be raised whenever needed to avoid the augmented second.
A third in a final harmony should be raised to be major if it is not already so.
There is no modulation in modal polyphony. You can write in a transposed mode,
but the mode should remain the same throughout and with the same final. That is, it would not be stylistically appropriate
to move from D Dorian to G Dorian, or from any mode to a different one.
The usual reasons for altering a mode tone imply that the altered tone will be left by
step. It is unusual for an altered tone to be departed by leap, but these will be allowed from the lowered upper tone of the tritone. That
is the tone most often altered to avoid a harmonic, melodic, or outlined tritone. Sometimes a necessary lowering of that tone produces a situation in which a
nearby voice has the same tone in the unlowered form. That other tone must then be lowered itself to avoid a cross-relation, regardless of its
movement.
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