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Originally Posted by BuddyManx
Thanks for that Kirk- It is the "several possibilities" that have got me thinking- as I've been looking at a couple of Real Book versions of "After You've Gone" which have a number of differences in the chord progression-which made me wonder about just how much latitude you have in the choice of more outside notes.
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However a post by Fretsource on why chords are bulit in thirds sort of cleared that up:
Chords are built on thirds because they're the smallest consonant (well matching) intervals. Seconds are smaller but clash because they're dissonant, not consonant.
Originally harmonies were built from two-note perfect fifth intervals, but fifths are a bit too pure and bland so they started sticking an extra note halfway between the two notes of the fifth - The new note was a third above and below the notes of the fifth. They sounded good and had an edge that the pure fifths didn't have - so they became standard.
After that they had the idea to start extending the chords by adding more thirds above the fifth and so 7ths 9ths and other extended chords were born. This type of harmony is called tertian harmony because it's made from thirds.
But there are other types of harmony built from seconds and fourths, called secundal harmony and quartal harmony. Modern classical composers experiment with those types of harmonies but most people find them unlistenable because, unlike chords built from thirds, they're ALL dissonant.
SO in choosing chord tones we are selecting for consonance. A penny has dropped I think.