The chord C Eb G Bb D F A would typically be notated Cm13. However, it makes a convenient example for this question.

Suppose I want to write that same Cm13 chord as a polychord. I have two choices: BbM7/Cm or Dm/Cm7. Would the choice of spelling have implications for the voicing of the chord or the musical meaning of the chord (such as harmonic function)? Or would either one simply be taken to express a set of notes to be played in a certain presumptive bottom-to-top order (i.e., as equivalent)?

There are other interpretations of the notes in the chord. It could be revoiced, for example, as F/EbM7. For the purposes of this question, I'd like to stay out of the weeds of all the different ways an unordered set of pitches could be written as a polychord.

  • No, it matters very much. Even the base of the chord or its inversions make a very different impact and sound. Also, many a times, chords are written in other forms, for example B7 is also written as Cb7, to avoid unnecessary modulation and stick to the key. – Ishan.J Mar 25 at 5:07
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    Possibly relevant: if any of you here encountered a polychord symbol, how likely would you be to omit the fifth of any seventh chords written within that polychord? In the case of the symbol Dm/Cm7, would you ever consider omitting the note G? Neat question. – user45266 Mar 25 at 5:24
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    @user45266 If you were to arrange it on a six-string guitar... likely something would get omitted. – user1079505 Mar 25 at 5:56

To me both spellings would yield very similar results in most situations BUT the look of Bbmaj7/Cm is awkward because you have a more complex chord, a 7th, on top of a triad. A common way of voicing chords with a lot of tensions like a Cm13 is just what you wrote, Dm/Cm7, a triad containing the upper tensions on top of a basic left hand voicing, for example: C Eb G Bb in the left hand and any inversion of a Dm triad in the right with a bit of a gap in between. The triad can also have the top note doubled an octave below. The left hand chord can also be rootless and/or have no 5th although I actually prefer having a 5th in minor chords.

Most experienced jazz players will consider Cm13 and Dm/Cm7 to be the same thing but the poly chord spelling will almost certainly get you a voicing with the triad sound on top whereas the Cm13 might get you that or something else, like a cluster voicing.

  • This presumes piano. What would happen on guitar? – Tim Mar 25 at 7:17
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    @Tim As always with 6-7 note chords on guitar, you run out of fingers and/or strings and have to decide what to play and what to leave out. I personally don’t write chord symbols like Dm/Cm7 unless I really want a triad sound on top. – John Belzaguy Mar 25 at 7:32

This is going to be very subjective :)

To answer the second part of your question first - An F/EbM7 would imply an Eb on the bass, which does not reflect the intended Cm13. So in that way, the interpretation does matter.

In terms of how to group the notes (i.e. where to put that Bb), I think the rest of it is up to harmonic function, and what might be easiest for the performing musician to read.

If there are important notes you want to make sure are played in the chord, this could have an effect as well. e.g. a performer may drop the 5th if they see a 7 chord.

If I had to pick one of them, I would probably prefer Dm/Cm7 personally, as I picture building up from the bottom chord, and adding 3 notes (Dm) above Cm7 is easier in my mind than adding 4 notes (BbM7) above a Cm.

  • Good point; jazzers are usually accustomed to thinking about chords as relative to basic seventh chords, so having the lower one be a seventh chord might be better in a lot of cases. – user45266 Mar 25 at 5:22

Actually, you have more choices than that. A 13th represents a full cycle of notes-- a 15th would be back to the tonic. That means you could take ANY 3 consecutive notes in your chord and isolate them: Gmin / F7 (Or F7 / Gmin if you like) E♭Maj / dmin7 and so on.

I think it's up to YOU how you want to spin this narrative. I would only spell it as a polychord if one group were relatively stable, and the other group were highly dynamic. For example: Dmin / Cmin7 -> G7 / Cmin7 -> A♭maj7 / Cmin7

In this narrative, you actually have a fairly traditional harmonic motion playing out over a stable collection of notes. But oh what a harsh dissonance that G7 will create!

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