Today I was playing Over the Rainbow - as a jazz ballad. Running up the chord to a big Eb t the end both my hands are over middle C and ready to play triads an octave apart.
Standard stuff and of course it will do, but...

I wanted to put a bit of uncertainty into it, just a question begged at the end, nothing too drastic. I tried various options and then settled on replacing the third in the left with a ninth. For me ninths and sixth are closely sonorous and can easily be exchanged. This note creates a little damage to the integrity of the major end, but just that little I was seeking.
I can feel that ninths and sixths are diatonic and although not stated in the chord are pretty straightforward alternatives. Ninths, sixths, then what? I don't have a hierarchy for selection, concord to discord.

For me the major seventh sticks out big time, its the leading tone begging for the octave a very strong pull, not at all like its sister the dominant 7th. Using s major 7th to replace dominant 7th or vica versa. This again is a question of classifying dissonance, most text point out that the dominant seventh has a triton built from third to 7th - fair point, but to my ears a major seventh seems more dissonant. Why is this so?

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