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I'm really curious about the difference between polychords and extended chords. I know a polychord is two chords played at the same time where there is a higher chord and a lower chord. The common example I see C/Dm when a polychord is demonstrated, however the way I look at is just a Dm11 chord that is voiced with a closed position Dm in the lower octave and a closed position C in the higher octave.

Is there a more distinctive difference between polychords and extended chords or are they just specially voiced extended chords?

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​​​​​​​​​​​​​​If I have a sequence of notes as a chord, say, C# E G B, I could call that chord Em6 or C#m7b5 (or even C#dim#7). For most series of notes there will be multiple ways you can notate it. It's the same thing with polychords - polychords can be notated as extended voiced chords (e.g. Db/G = G7#11b9) if you want, but sometimes the polychord notation is just easier to read at a glance. – Pyrrha Sep 3 '14 at 7:04
up vote 6 down vote accepted

The polychords use a fraction for a symbol, like: enter image description here, so as to distinguish from the slash chords.

From what I have understood, the point of the polychords is to help the player read and play faster. It is easier to read Abm (fraction) G7 rather than G7b9b13. The following chord can be called both G7b9b13, and Abm (fraction) G7.

enter image description here

(I know that someone can say -- Hey, just practice and you'll be able to play it -- but, it exists)

Quoting Jazzology page 15 (Chapter 2):

In practice, jazz pianists and arrangers prefer simplifying upper-structure notation so as to make the chords easy to read and play.

According to the Wiki article on Polychords, it is also used for bitonality or polytonality.

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The chord you use as an example is not exactly Abm over G7, because B and D are omitted. A polychord suggests full voicing (no omitted notes), and that very trait could be defining for the difference between a polychord and an extended chord. – SeuMenezes Oct 23 '14 at 22:01
One correction: B is not omitted, as Cb is an enharmonical equivalent to B. Nonetheless, D should still be there to form a full polychord. – SeuMenezes Oct 23 '14 at 22:25

You are right that in principle there's no difference other than that a polychord has two complete triads, which is of course not necessary for a general extended chord. Obviously, polychords are related to upper structure triads. I would say that polychords are a special case of extended chords which contain two complete triads. The main advantage is a notational one. A polychord is a simple notation for a complex chord structure.

BTW, note that polychords do not necessarily consist of two triads in closed position as you seem to imply in your question.

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A polychord indicates an explicit superimposition of two or more chords, with full voicing, disallowing the omission of any notes. It is usually used to simplify the notation of an otherwise awkward chord symbol [think of Ebm7 over G, which could result in a bulky GMaj7(#9#11b13)], but can also make clear that the composer was thinking specifically about chord superimpositions, instead of chord alterations.

It is worth noting that a polychord can be also used to avoid the forced attribution of any harmonic function to a chord when there is no strict tonal context (for instance, a cluster, a non-tertian chord or a passing chord).

A polychord can be voiced in any way you want (open, close, interlocked or not), depending on any specific texture you intend to create.

Finally, it is rarely necessary to try seeing chords as inversions of other chords. One should trust the composer/arranger's initiative to notate a chord in such deterministic way, as to give an exact character, to influence the voicing of the arrangement, the conduction of the bass voice, and many other traits of the work being played/analyzed.

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