The_Complete_Idiot%27s_Guide_To_Music_Theory

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Part 5: Embellishing
As you can see in the following table, some of the same substitution rules work
with minor as well as major; especially the “up a third” and “down a third” diatonic
substitutions.
Minor Chord Substitutions
Substitution Example (for the A minor Chord)
Major chord a third above
Major chord a third below
Major 7 chord a third below
Diminished chord with same root
The last substitution falls into the “more of a good thing” category. That is, if a
minor chord sounds good, let’s flat another note and it’ll sound even more minor.
Some folks like the use of a diminished chord in this fashion; others don’t. Let
your ears be the judge.
Dominant Seventh Substitutions
Okay, now you know how to substitute both major and minor chords; but what
about dominant seventh chords? They’re not really major and they’re not really
minor—what kind of chords can substitute for that?
The answer requires some harmonic creativity. You can do a diatonic substitution
(using the diminished chords a third above or below the dominant seventh), but
there are more interesting possibilities, as you can see in the following table:
Dominant Seventh Chord Substitutions
Substitution Example (for the G7 chord)
Major chord a second below
Diminished chord a third below