Roo Stercogburn Wrote:...to good musical effect?
If so, what is the music theory. Back up your answer with musical evidence.
Evidence is easy, though I'm not sure about the music theory. The notion of a key rather breaks down if you're thinking about using two or more at once.
For instance, you can easily put a minor chord on top of a major chord, but then mostly what you've got is a major chord with notes on top; e.g.
C major + E minor is a major 7th with C as the root: C + E + G + B
C major + G minor is a 9th with C as the root: C + E + G + Bb + D
More obscurely, C major + Eb minor is a nice crunching chord again based on C (i.e. C + E + G + Eb + Gb + Bb) — this is the final chord in Poulenc's
Sonata for Piano Four Hands
(though the video is a bit annoying because it cuts off really quickly, whereas the chord should be left to ring out).
Polytonality (playing in more than one key at once) is more unusual. The clearest examples I can think of are Milhaud's
Saudades do Brazil. Here are three of them:
The basis of the first piece is G major (left hand) / B major (right hand); the second is Eb minor (left hand) / F major (right hand) – though they modulate into other separate keys as well.
There's another big polytonal crunch in Stravinsky's
Rite of Spring:
The big riffing starting at 3:38 is an Eb 7th major chord on top of an E major chord. Sounds great doesn't it?
But theory, I dunno. We need UFO back here again. He could probably tell you about theory.