Modal Interchange & Altered Chords

Using Modal Interchange to Alter Chords

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Modal interchange is a system that helps us to create interesting and musically relevant chord alterations. This is another type of chromatic chords, which means that one or  more note is not in the actual key signature or implied key centre.

We have already discussed two types of chromatic chord

  • Secondary dominants – which are often used in jazz to create chromatic interest.
  • We have also seen how upper extensions can be altered:
chromatic alterations
ex 4h: Note that the #9 and b10 have the same notes. The C7 augmented and C7 b13 have the same notes.

Jazz musicians often reharmonise standard tunes not only by altering chords but by using substitute chords. When an established melody is being played the choice of these new chords is restricted by the melody note, however during improvised solos there is far more freedom.

As mentioned earlier – see Minor Harmony – chords can be borrowed from other modes. This is sometimes called modal interchange and is common between a major key and its parallel minor. (not the relative minor which has the same key signature, but the minor key which has the same tonic). Chords can be borrowed from the harmonic minor, melodic minor or any minor mode. Here are some of the more common examples:

1) Substitutions for V7.

bVII7 can be substituted for V7. This is borrowed from the Aeolian mode of C (based on major scale of Eb – Cm7, Dø7, Ebma7, Fm7, Gm7, Abma7, Bb7, Cm7), but used in a major key.

altered chords
ex 7a: Modal interchange using bVII (chord V of Eb hence chord VII of the Aeolian mode).

IVm7 (or IVm6) can also be substituted for V7. Also borrowed from the Aeolian mode (Cm7, Dø7, Ebma7, Fm7, Gm7, Abma7, Bb7, Cm7)

altered chords with aeolian
ex 7b: IVm7 modal interchange using the Aeolian mode

Combine these two and we have a II-V progression borrowed from the Aeolian mode

altered chords with aeolian
ex 7c: IVm7 – bVII7 borrowed from Aeolian mode. Note the alternative RN Analysis

From the above we can see that a IIm7-V7 in an apparently unrelated key (i.e. IIm7-V7 in Eb can be used in the dominant area of the key of C (see Misty bar 4).

bVII maj7 (from the Dorian) can also be used in modal interchange but does not lend itself to the IIm7-V7 progression in ex 7c, and is not such a good substitute where a perfect cadence is implied.

2) Substitutes for IVmaj7

By borrowing from the Aeolian mode again, IV maj7 (or 6) can be altered to IV m maj7 (or 6) or IV m7. In popular music of the 30’s, 40’s and 50’s IVm was often used following a IV and preceding a I, adding passing notes to a plagal (IV-I) cadence.

altered chords modal interchange
ex 7d: Modal interchange supplying passing notes on Ivm – (1) D and Ab (2) Eb and Ab.

 

Modal Interchange & Altered Chords
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