Will two versions of a song, in the same key, fit together if supporting chords differ?

Question

I get that the title is vague, but that's kind of the problem. Basically, I've learned a jazzy version of the song "Heart and Soul" on the piano (you know, the one everyone learns to play with someone else). For reference, here's the sheet music (keep in mind, the first 6 bars are just jazzy stuff, pretend it starts on bar 7.

Someone asked me "if someone was playing the 'normal' [that is, the C Am F G version] of the song, and you played the jazzy version, would they 'fit together'?" I tried to explain that no, because for example halfway through bar 7, I play an F7 chord, while the standard version would play an Am chord, and these two chords are at odds, but she says "Oh that just sounds jazzy, not wrong". She hasn't played piano for years and doesn't know terminology so I couldn't get her to say what she meant in musical terms, but basically, she says that "If two people play heart and soul [that is, the same melody], and play chords 'in the key of c', then their two songs will fit together." I couldn't get her to explain what she meant by "In the key of C" though, whenever I probe she just says "Well I don't know what the musical terms technically mean". Basically, can you confirm that just because two people play songs with the same melody, and chords which go with that melody, their two versions don't have to align? Or am I crazy here?

Answer 1

Usually, different harmonisations will clash badly, as you say. Sometimes, by chance, they might fit well enough to give something that could be heard as a coherent “jazzy” harmonisation. If they’re only very slightly different, this could reasonably happen; but if they differ significantly, this is very unlikely.

Specifically: wherever the harmonisations differ, their combination will give a superposition of two chords. Often, the superposition of two simple chords will give something that can be seen as a single more complex chord: for instance, F major and a minor chords, together, create an F major 7th chord; or F major and c minor together make an F dominant 9th chord. So especially if the two original harmonisations differ in only a few places, it’s possible that (by luck) the complex chords created in those places will be good chords that happen to fit in with what’s around them, so that the combination can be heard as a single “jazzy” harmonisation.

More often, though, if the harmonisations differ significantly, some of the chord-combinations they create will be much more clashing — e.g. F 7th and a minor together (as in your example) give F major+minor 7th, which is very dissonant and quite rarely used. Or, even when the individual combination chords created are reasonably chords, they may not fit together in a sequence that makes harmonic sense or fits the song. For instance, if one harmonisation shifts between F major and a minor on alternating bars, and the other does the same but the other way round, then the combination will give F major 7th on every bar — so the shifting effect will be lost. Indeed, this sort of problem is most likely: just like choosing random chords is very unlikely to make a good harmonisation, it’s unlikely that the chords “randomly” created by combining two harmonisations will give something that works well.

Answer 2

Maybe you could record one version, and replay it while you play the other! At that point, you'll realise that, no, they don't fit well with each other.

While the melody itself is the same - although the timing has been tampered with - the harmony for the jazzy version differs from the original, so the two cannot fit over each other. It's always possible to re-harmonise pieces, it happens a lot in jazz, and that's what's happened here.

Two or three notes in a particular bar can have several different chords which fit, so unless that side of the piece hasn't been altered, the two won't fit well. Merely being in the 'same key' isn't enough.

Answer 3

You cite bar 7, where one version uses Am, the other uses F7.

Am includes E♮.. F7 contains E♭. They will clash. You MIGHT be able to convince yourself they combine into one super-jazzy chord, but I doubt it!

There are other similar examples. You are right, your friend is wrong.

I expect you have some method of recording one version so you can play the other against it? Do so and prove the point.