The octave rule, major ascending

Rising through the octave, the standard harmonization gives the tonic its 5/3, the second degree 4/3, the third degree a sixth, the fourth degree 6/5, the dominant its 5/3, the sixth and seventh degrees sixths, and the tonic its 5/3 again. The logic is directional: each mobile degree carries the chord that pulls it onward — the 6/5 on the fourth degree pressing toward the dominant, the sixth on the seventh passing up into the tonic.

The ascent is the workhorse of openings and sequences — basses climb from tonic to dominant constantly, and the Rule supplies the inner voices without deliberation. Knowing which degrees carry motion chords also explains the repertoire’s habits: the 6/5 on the fourth degree is why so many rising basses arrive on the dominant with such conviction, and the sixth on the seventh degree is how a bass walks into the tonic without a cadence.

• Giving the seventh degree a root-position triad, putting the leading tone’s diminished chord in a bass position it cannot carry.
• Doubling the leading tone or the bass of a sixth chord while realizing the pattern at speed.
• Forgetting the 6/5 on the fourth degree and losing the pull that makes the dominant an arrival.
• Letting the upper voices march in parallel with the bass instead of trading contrary and oblique motion against it.

• Fedele Fenaroli, Regole musicali per i principianti di cembalo (1775), The rules for accompanying the scale (ascending) — The Neapolitan per-degree assignments for the rising bass.
• François Campion, Traité d’accompagnement et de composition selon la règle de l’octave (1716), The règle de l’octave — The original codification the later schools enriched.