TromboneChat

I suspect I know the answer to this question but would like the opinion of someone more learned in jazz theory than myself:

I think I understand that the altered scale (7th mode of melodic minor) is substituted for the  5th mode as the "V" of a minor ii/v/1.
But why is the half diminished scale  (6th mode) substituted for the susb9 (2nd mode)? why is it still called a ii/5/1 instead of a 7/6/1 ??

Thanks professors.

The raised 6 in the melodic minor scale gets you two harmonic things: the perfect fifth in the ii chord (instead of it being diminished) AND the major third of the IV chord. And, I guess, the diminished vi chord, too, if you want to take advantage of that, I guess - but that's less commonly used.

Also consider that in melodic context the whole "use the lowered version of those two notes" tends to happen somewhere around the beginning/middle of a phrase, and the "raised" version tends to get used toward the end of the phrase. Hence, the harmony tends to stay more true to the original mode until the end of the phrase, where the "raised" melodic notes support the expected ii-V-i cadence.

The raised 6 in the melodic minor scale gets you two harmonic things: the perfect fifth in the ii chord (instead of it being diminished) AND the major third of the IV chord. And, I guess, the diminished vi chord, too, if you want to take advantage of that, I guess - but that's less commonly used.

Also consider that in melodic context the whole "use the lowered version of those two notes" tends to happen somewhere around the beginning/middle of a phrase, and the "raised" version tends to get used toward the end of the phrase. Hence, the harmony tends to stay more true to the original mode until the end of the phrase, where the "raised" melodic notes support the expected ii-V-i cadence.