Scales as Chords

I'm looking at the chord-scale system and polychords as a shortcut for coming up with nice Jazz voicings.

Example

We start with a scale, for example, C Lydian.

C Lydian scale

Playing all the notes of this scale at once, we hear a big Jazz chord. We'll give each note some space to breathe by stacking thirds.

C Lydian as single chord

We can decompose this chord into overlapping triads.

C Lydian overlapping triads

Playing Em, G, Bm, or D (possibly in some inversion) over C gives us polychords that correspond to the following chords with a few notes doubled.

Polychords from overlapping triads

In terms of polychords these are much easier to write down and remember. Of course, each chord may have different voicings: we simply play different inversions and leave out or double notes as we see fit.

Chord voicing examples

Generalization

To generalize over key we can use Roman numeral analysis. I'm using the traditional notation based on the major scale, so flats and sharps indicate deviations from that scale. For the Lydian mode, the overlapping triads are I (C), iii (Em), V (G), vii (Bm), and II (D). For all seven modes, we get the following table.

Mode 1 3 5 7 9 Chord
Ionian (major) I iii V viio ii I▵13
Dorian i ♭III v ♭VII ii i13
Phrygian i ♭III vo ♭vii ♭II i7♭9♭13
Lydian I iii V vii II I▵ ♯11 13
Mixolydian I iiio v ♭VII ii I13
Aeolian (minor) i ♭III v ♭VII iio i7 ♭13
Locrian io ♭iii ♭V ♭vii iio i7♭5♭9♭13

As a little exercise, pick a mode and a polychord and move through the cycle of fourths on the piano.