The Rule of the Octave

The Rule of the Octave is the thoroughbass player’s default map: harmonize a scalewise bass with 5/3 chords on the stable degrees — the tonic and the dominant — and chords of the sixth on the mobile degrees between them, enriched at the second, fourth and seventh degrees according to the direction of travel. When the figures are silent, the Rule is what the accompanist plays; when a partimento bass moves by step, the Rule is the harmony it implies. Masters differ in the enrichments — plain sixths in the simplest versions, 4/3, 6/5 and 4/2 in the fuller ones — but the skeleton is fixed: stability at the pillars, motion between them.

It is the first thing a continuo player memorizes and the fastest harmonization a student can produce: with the Rule in the fingers, any stepwise bass plays itself, and attention goes to the exceptions the figures mark. It is also a diagnostic frame — progressions that feel inevitable over stepwise basses usually are the Rule, and recognizing it by name turns a page of figures into a handful of decisions.

• Harmonizing every degree with a root-position triad, which turns a line into a lurching chord drill and breeds parallels at every other step.
• Treating the Rule as unbreakable law rather than the default the figures override.
• Applying it to a leaping bass, where root motions, not scale stations, decide the chords.
• Mixing versions mid-scale so the enrichments contradict the direction of travel.

• François Campion, Traité d’accompagnement et de composition selon la règle de l’octave (1716), The règle de l’octave — The treatise that named the Rule and set the per-degree map.
• Fedele Fenaroli, Regole musicali per i principianti di cembalo (1775), The rules for accompanying the scale — The Neapolitan schooling of the same map, drilled in both directions.