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Probably this is a matter of personal taste, but I thought I might ask to be sure that there isn't a more in-depth explanation out there.

To my ear, 9th chords and minor 7th chords are oddly interchangeable. I do not understand why this may be the case — no other two chords built on the same root sound so similar to me. This goes entirely against my intuition, especially because of their respective major and minor thirds.

Does anyone else notice this, or is it just me? Is there a theory explanation for this?

I think there may be a connection to how tritones tend to be resolved inward or outward by half-step (for the sake of stability); the major second and third are a half-step to either side of the minor third.

PS: For the record, I am a guitarist and usually voice the chords 1-3-♭7-9 and 1-5-♭7-♭10(♭3).

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  • @DavidBowling I'm speaking more about chord quality than function. You can think of it being used as the I chord though. – Phoenix Jul 29 '19 at 20:58
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    1-3-7-9 (in key C) = C-E-B-D. 1-5-7-b10 (#9) (in key C) = C-G-B-D#. Are they the notes you really mean? C9 is normally C-E-G-Bb-D, and Cm7 is C-Eb-G-Bb. This is confusing, Unless you mean the roots of each are different, thus making different named chords. – Tim Jul 30 '19 at 5:32
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    If you can't hear the difference when notes are being played harmonically, you might consider playing them arpeggiated until your ears pick it up. I've had a similar (albeit different) problem with learning be-bop scales, and practising them slowly over and over again did the trick. Some people do indeed have trouble differentiating chords that are larger than a triad. I think it's pretty common. – Pyromonk Jul 30 '19 at 11:43
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    @PhoenixMorrison, I had thought you were asking about inversions or substitutes. For example every Maj7 chord has a minor triad in it and some people hear the minor content of the Maj7 but if hear A9 and A-7 as the same and you aren't eliminating the 3rd then you need some ear training. As previous comments state, arpeggiate until you hear the difference. It is possible that your ear is just focusing on one interval in each chord or some subset of intervals but A9 and A-7 are not very similar imo. – ggcg Jul 30 '19 at 11:51
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    @Pyromonk Thanks for the advice! I can see how it is most likely a problem of ear training. I still don't understand why I would have this problem between these two chords in particular (even amongst chords larger than triads). – Phoenix Jul 30 '19 at 18:06
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I think it's important that you specified...

...built on the same root

...and that you acknowledge...

...This goes entirely against my intuition, especially because of their respective major and minor thirds.

...because they don't sound interchangeable to me.

But, what similarity could we find?

First, let's take your specific voicings. The can both be seen as two pairs of similar intervals...

G9

 M3    M3
 /\    /\
G  B  F  A

...two major thirds separated by a minor seventh.

Gm7

 P5    P4
 /\    /\
G  D  F  Bb

...two perfect intervals separated by a minor seventh. Unlike the G9 they aren't the exact same interval, but they are both perfect and of course the perfect fifth and perfect fourths are inversions of each other.

So both chords in these voicings are a sort of composite of two exact or similar/inverted intervals.

But, I think there is a more interesting similarity.

If the fifth of G9 is sort of swapped for the third of Gm7 there is similar combination of intervals between the two chords...

G9

Root G -----|
            |
7th  F \    P4
        m3  |
5th  D /----|

Gm7

3rd  Bb\----|
        m3  |
Root G /    P4
            |
7th  F -----|

...if we disregard the specific chord tone identities those two are an inverted pairs. Of course when the chord tones are considered the third is absent from the G9 group, so it's sort of a 'cheat' to say this makes the chords interchangeable.

If you omit the third from G9, you can compare voicings like G D A (omit B) F and G D Bb F where there is a similar stacking of perfect fifths.

The similarities of the intervals and inversions above of course are dependent on the specific voicings. If the voicings are changed, I think you will loose the possible similarities.

From a harmonic voice leading perspective the move of scale degree ^4/FA to ^3/MI (or MA for minor) is fundamental to define the tonal center, the key. Basically, ^4 resolves to ^3 especially when ^4 is part of a dominant chord.

Maybe if the harmony style somehow avoids the tonal resolution to the third - or if the harmony switched between the major and minor third in an ambiguous way - these two chords could be interchangeable.

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  • Thanks! I have to spend more time digesting this, but super interesting stuff. – Phoenix Jul 30 '19 at 18:31

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