The octave rule, minor descending

Coming down, the minor mode reverts to its own scale: the seventh and sixth degrees are natural, and the chords above them are the minor dominant and the minor subdominant in first inversion. Only the dominant itself keeps its accidental, and only where it functions as a dominant — on the fifth degree, and in the inversions of that chord standing over the fourth and second degrees. The descent is where a student learns that the raised tones belong to a chord, not to the scale.

Falling minor basses are everywhere in the repertoire — laments, ground basses, the approach to any minor cadence — and this is their default reading. It is also the clearest demonstration of a rule students find slippery: the seventh degree is natural when the harmony above it is the minor dominant, and raised when the harmony is the true dominant, so the accidental follows the chord rather than the scale degree.

• Raising the seventh degree on the way down out of habit, which turns the minor dominant into a chord the descent never asked for.
• Failing to resolve the 4/2 on the fourth degree, when the bass is itself the chordal seventh and must step down into the sixth chord below it.
• Doubling the raised third of the dominant wherever that chord appears — on the fifth degree, and in its inversions over the fourth and second.
• Reading the accidental as belonging to the scale rather than to the dominant chord, and then applying it where no dominant is sounding.

• Fedele Fenaroli, Regole musicali per i principianti di cembalo (1775), The rules for accompanying the scale in the minor mode — The falling minor rule, with the dominant’s accidental reserved to the dominant.
• François Campion, Traité d’accompagnement et de composition selon la règle de l’octave (1716), The règle de l’octave — The two-directional map, minor as well as major.