Ah. Just now reading your other replies. Unfortunately it's subjective, fuzzy, and often rooted in convention rather than following algorithmic rules. An algori

Ah. Just now reading your other replies. Unfortunately it's subjective, fuzzy, and often rooted in convention rather than following algorithmic rules. An algorithm is probably impossible in the general case. In the case of jazz standards (and assorted other genres like Western popular music), melodies are often published alongside the author's intended accompanying chords, so when those are present they could maybe be used as a training set for an automated solution. But for other genres, a ground truth label might exist only in the author's mind, or in the minds of sophisticated listeners familiar with that genre. Reasonable people might disagree about the fact of the matter in some cases (or shrug and say there is no fact of the matter and either option is okay). Sometimes when there's more emphasis on melody than harmony, the choice of C#/Db might come down to ease of reading (ascending notes B-C-Db might be easier to read than B-C-C#) rather than harmonic considerations. Or maybe as another commenter points out, the reverse convention might be used.