Chord Substitutions

by Jim
(Charlotte NC)

Question :

Is it always safe to substitute a Am7 for a C6 since they contain the same notes?

How much does the chord voicing matter in the m7?
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Answer :

Any of theses subs can be used anywhere and anytime... EXCEPT when the voicings has notes in the lower register. So the chord voicing matters if you want the Am7 to sound like so (and not C6!)

The thing is, the bass note really "defines" the function of the chord. If you are aiming to play a C6 and the "A" note is the open 5th string (fairly low register), it will sound like a Am7... That is, if you are playing by yourself.

If another instrument (say the bass), is playing a low C note, then it will simply "clash" with it.

Of course, they're are no strict rules because some chords with voicings in the low register can work perfectly as passing chords, for instance. You have to experiment to hear this. Also, I remember reading about "safe low limits in the bass register" in arranging/composition books and in Bill Leavitt books. Worth investigating...

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Another consideration in voicings, inversions and substitutions is the presence of "flat 9" intervals. It's not related to your C6 = Am7 situation, but I'd like to share :

What happens if you play a Cmaj7 chord in a inversion that puts the "C" note on top and the "B" note on the bottom?

The interval of a minor 9th...
(= an octave + a 1/2 steps).

This makes the major chord to sound "eery" an sometimes plain ugly. Watch out! It happens in chords with a major 7th but also in certain extended chords. Whenever something implies a "half step" (for example, above "B-C"), it can lead to trouble!

I hope this helps,
Marc-A