We have discussed secondary dominants and tritone substitutions as effective ways of enhancing basic chord progressions. Another common method of accomplishing this is through the use of modal mixture or the technique of mixing harmonies found in two or more modes. The most common method is to mix the diatonic seventh chords from the major (Ionian) mode with chords from the parallel minor (Aeolian). Be sure you are clear about the distinction between parallel minor and relative minor. Parallel minor has the same tonic tone as the major scale we are comparing it to. In other words C minor is the parallel minor to C Major. The relative minor to C major would be A minor. Let’s review the chords in both C major and C minor.
Diatonic seventh chords in C Major: CMaj7 Dm7 Em7 FMaj7 G7 Am7 Bm7b5
Diatonic seventh chords in C Minor: Cm7 Dm7b5 EbMaj7 Fm7 Gm7 AbMaj7 Bb7
So far so good, now let’s mix them. We will start with the most common. It is a sound I am sure everyone is familiar with if you’ve ever listened to Beatles tunes; the Minor iv chord used in place of the Major IV chord. C – F – C becomes C – Fm – C. It is also common to use both forms: C – F – Fm – C. Introducing that one new note (Ab) adds color and resolves beautifully back to tonic as the Ab will have a strong pull downward to the G or the fifth of the tonic C Major. In jazz we would more than likely add the seventh to these chords making the progression CMaj7 – Fmaj7 – Fm7 – Cmaj7. By the way, these altered chords are usually referred to as “borrowed chords” since they are borrowed from the parallel minor scale.
Along with the Minor iv, altering or borrowing the ii chord from the minor scale in a major key is very common. Dm7 – G7 – Cmaj7 is often changed to Dm7b5 – G7 – Cmaj7. Again the introduction of the flat sixth scale degree (Ab) will add color and move nicely to G.
The use of the bvi chord (AbMaj7) and the biii chord (EbMaj7 ) in a major key in various combinations is also very effective: CMaj7 – AbMaj7 – G7 – Cmaj7 or CMaj7 – EbMaj7 – AbMaj7 – G7 – Cmaj7 are beautiful progressions.
The use of the bVII in a major key is another great sound: CMaj7 – Fmaj7 – Bb7 – Cmaj7 for instance works beautifully and is often found in many jazz standards.
The Minor v chord (Gm) in a major key is least common since the usual dominant seventh with all of its possible alterations, which by the way are derived by adding tones from the parallel minor scale, is by far the best choice for additional color. For instance if we look at the altered dominant in the key of C major, G7(b9#9)(b5#5), three of the four possible altered tones are derived from C minor; the b9 (Ab), the #9 (A# or Bb), and the #5 (D# or Eb).
There are even more choices available when we combine chords derived from the harmonic and melodic minor forms of the minor scale with chords from the parallel major scale. The most common of these would be the use of the fully diminished seventh chord to replace the half-diminished (m7b5) chord on the seventh degree: Bm7b5 – Cmaj7 becomes Bdim7 – CMaj7. Remember that this chord (Bdim7) is best thought of as a rootless dominant seventh with a flat nine; in this case G7b9.
In summary, when you come across a chord in a tune that isn’t a secondary dominant or a tritone substitution and still doesn’t seem to fit in with the usual diatonic harmonies ask yourself if it could possibly be a borrowed chord derived from the parallel minor scale. There is a very good chance it is.