The octave rule, minor ascending

The minor ascent follows the major’s plan — stable degrees taking 5/3, mobile degrees taking a sixth — with two differences the mode forces. The sixth and seventh degrees are raised on the way up, because a bass climbing from the natural sixth to the leading tone would step through an augmented second; and the dominant needs an accidental to raise its third, since the mode does not supply a leading tone of its own. What the major mode gets from its scale, the minor mode has to ask for in the figures.

Any minor-key bass that climbs toward the dominant or the tonic wants this harmonization, and it is the pattern that teaches why minor carries accidentals the major mode never needs. It also settles a question students otherwise get wrong by ear: the raised sixth degree is not a modulation or a borrowed colour, it is the repair that keeps the ascending bass singable.

• Leaving the sixth degree natural on the way up, so the bass steps through the augmented second the raised sixth exists to remove.
• Doubling the raised seventh, which the figures introduce precisely once and which must rise to the tonic.
• Writing the accidental into the realization but not into the figures, so a player reading the bass has no way to know it is there.
• Carrying the raised sixth and seventh back down again, where the descent has no need of them.

• Fedele Fenaroli, Regole musicali per i principianti di cembalo (1775), The rules for accompanying the scale in the minor mode — The Neapolitan schooling of the minor rule, drilled in both directions like the major.
• François Campion, Traité d’accompagnement et de composition selon la règle de l’octave (1716), The règle de l’octave — The original codification, which sets out the minor alongside the major.