All of these 6ths are just add a mildly dissonant note to what are functionally triads -
so they are a flavour rather than a functional extension (unless you consider that for
instance a dm6 is just a 2nd inversion Bm7) If you're interested in functional 6th look up
the augmented 6th in its various flavours (French, Italian, and German) - you don't hear
that in many pop songs.
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Daniel Davis
Edinburgh Recording Studio Windmill Sound