Polychord harmonize

my guess would be to keep it in key... so if you play a C major chord, play a another chord that's in the same key... so in C major you could play:

D minor, E minor, F major, G major, A minor, B diminished (although I don't know how well a diminished chord will sound)

As well, chords that have one or several chord tones from the original chord would probably work best. So if you had C major (C E G), I think that Am7 would be good (A C E G), Em7 (E G B D) would be an interesting one too and would give you more colour then the Am7. Of course I'm sure chords outside of the key would work too, I don't have much experience at all with composing in this sort of complex manner, just experiment to see what works.

It's a tough question, because the answer is really simple: play whatever fits.

When you're doing a polychord, by definition the result will be a four-note chord (sixths, add9s, or extended chords) or larger, and often the result will be an altered extended chord.

So you need to figure out what sort of substitutions you want, and go from there. Let's say the original chord is C, and you want a 'sweet' sound like a Cmaj7 or Cmaj9. You're now looking for (new notes in bold)

C-E-G-B or C-E-G-B-D

So now you get to play a chord that has the 'new' note... and overlaps the C chord. That gives you lots of choices, but the simplest two are:

E-G-B = Em
G-B-D = G major

Or maybe you're getting ready to transition to C7 as the main chord, and you want to anticipate that. Changing to Eº or Gm will result in C7 or C9, because the Bb note in your chord will lead to a dominant result.

There's no requirement to stay in key, or to have overlapping tones - they just make things easier to figure out, and keep the resulting chords 'small' - two triads without overlapping tones will give you an 11 or 13 chord, often with altered notes: Maybe you want to play Bb major... the result will be:

C-E-G + Bb-D-F = C11

or Db:

C-E-G + Db-F-Ab = 1-3-5-b9-11-b13 = C11b9b13

If you just try to 'wing it' on polychords, you probably won't like the results. In order to use them, you'll need a good idea of how chord alterations sound, and an excellent understanding of chord spelling.

Damn, I thought "Polychord Harmonize" must be some sort of furniture polish.

Furniture polish, method for embellishing chords and a dessert topping!

3 for 1!

tastes great... less trilling... tastes great... less trilling...