The term polymodal refers to the use of more than one mode at a time. Usually the layered modes share the same tonic, like C Dorian and C Lydian. Polymodal chromaticism is related to the fact that it is possible to obtain the chromatic scale, or parts of it, if you layer modes like the C Phrygian with C Lydian:
The above is the result of layering the presented modes, producing the chromatic scale. The notes in purple are the common tones from both modes
This approach provides new means to develop a melody, and harmony, by freely alternating between one mode and another while considering the notes from the resulting polymode as being of equal value.
You will be able to avoid, expand or develop a sense of major-minor tonality by developing melodies and harmonies that can be inspired by diatonic extension and chromatic compression.
Thus, polymodal chromaticism may be considered as an extension of diatonic scales. The following melodic excerpt illustrates the potential of this interchangeability using C Dorian and C Lydian:
Harmony Derived from Polymodal Chromaticism
When two modes are layered, the harmonic content that can be gathered is the one belonging to each tonality that can either be used simultaneously or not; and the resulting possibilities given by the polymodal expansion.
Picking up from the example above, if C Dorian is layered with C Lydian the resulting polymode has the following notes:
Layered modes with common tones represented in purple, in red the ones that belong to C Dorian and green to C Lydian
As mentioned, the resulting harmonic content is the same you would find in the respective tonal centers and also other harmonic possibilities that otherwise wouldn’t be possible to build if these modes were not layered.
Layered modes with possible chord combinations, also considering the enharmonic tones.
As you can see, we have the chords that belong to each modal center. However, because the modes are layered, we have resulting extra chords that don’t belong to either modal centers – represented in orange.
And because there are other alternatives provided by the extra notes, some chords associated to the different scale degrees may change their chord quality. Below is the melodic transcription of the previous example with the chord materials presented above:
All in all, this technique allows us not only to expand the melodic possibilities but also provides us with greater freedom and harmonic choice. The example below presents a polymodal melodic and harmonic approach, for A Aeolian and A Mixolydian, where both modes can be heard either simultaneously or separately by using polychords, chords from each modal center and chords with mixed material from layered modes, as contrast:
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