An Illustrated Guide to the "Rule of the Octave"

The chart below shows in abstract and greatly simplified form how eighteenth-century musicians conceptualized the relative stability or instability of the different scale degrees across an octave in the bass. The red boxes represent positions deemed stable points of arrival, and the green circles indicate positions felt to be unstable and more mobile.

As a first approximation of the rule of the octave, we can imagine that the stable scale degrees receive a 5/3 chord (as in do-mi-sol) and that the unstable degrees will take some form of a chord with a 6, perhaps 6/3 (as in re-fa-si). This simplified version highlights the great continuity in the traditions of polyphonic music, inasmuch as the chart below is almost indistinguishable from late Medieval and Renaissance presentations of fauxbourdon singing in cathedrals.

Like the melodic minor scale, the rule of the octave is not quite the same ascending and descending. So for a better approximation, let us examine movement up and down separately. Below is the ascending version. Dissonances (a clash between a "6" and a "5") were added to the scales degrees that precede the stable positions. So as one ascends the scale in the bass, maximum instability comes just before a return to stability.

The same principle applies when descending, though the dissonances are now between a "4" and a "3." In the descent from the sixth to the fifth scale degree, the "6" is raised a half step to create a leading-tone (F♯ in a C-major context) to the stable fifth scale degree, thus giving scale degrees two and six the same sonority.

There is still one more complication. The third scale degree was deemed partly stable, partly mobile. Following the principle of dissonance before stability, musicians often added a "4/3" dissonance to a rising second scale degree, and almost always added a "4/2" dissonance to a descending fourth scale degree.

"The" rule of the octave is thus not a fixed set of chords, but rather a summary or norm of the fluid and highly contingent practices of eighteenth-century musicians. In this series, the maestro Giovanni Furno details several further and more particular contingencies dealing with departures from simple scalar movement (click here to go to Furno's presenation). Both Durante, Furno, and Fenaroli present the basic rule of the octave, for both major and minor scales, in their respective Regole (Rules), and in this series their prescriptions are given both verbally and in musical notation.