If you study advanced harmony, you are likely to stumble across the term “polychord” sooner or later. A polychord is just the result of playing one chord on top of another.
Here is a simple polychord.
Note that we have a D major chord stacked on top of a CMaj7. You could call it a polychord, but I would not. I would call it an extended CMaj7 chord or something like that because that is what it really is. It is a CMaj7 with a 9th, #11th and 13th.
As you get into more complex harmony, labeling something as a polychord is not really very helpful. I would rather you see every note in relation to the root. In other words, you need to get to the place where a quick glance at the chord above tells you that it is CMaj7 with a 9th, #11th and 13th.
That being said, thinking in terms of polychords is a great cheat for helping you build complex chords quickly. For example, if I told you build a EbMaj7 with 9, #11 and 13, a very easy way to do it is to play an EbMaj7 and then play a triad starting a whole note up from Eb. If you play F Major on top of EbMaj7, you have your chord. You might change around the order of the notes of course and may leave a note or two out of course but that will not change what the chord is.
We call this kind of thinking polychord voicing. Here are a few of the more common polychord voicings and what they end up being in terms of an extended chord.
|Dominant||Major triad up a whole step||Dominant with 9, #11, and 13|
|Dominant||Major triad up a tritone||Dominant with b9 and #11.|
|Dominant||Major triad up a minor 6th||Altered dominant (#9 and b13)|
|Dominant||Diminished 7th up a half step||Dominant with b9|
|Minor 7th||Minor triad up a whole step||Minor 7th with 9, 11, and 13|
|Major 7th||Major triad up a whole step||Major 7th with 9, #11, and 13|
When you add these chords, remember that you don’t have to play them as a block chord. Let’s say that you want to play C7(b9). You could play C7 in the left hand and a Dbdim7 as an arpeggio in the right hand like this:
So this is the moral of the story. Polychords are a great cheat to easily play extended chords but they are a bad way of labeling chords. If you use them, always know what you are playing in relation to the root so you can understand the function of the chord.