The Rule of the Octave and Harmonizing the Scale with Sequences

The Rule of the Octave is an 18th-century formula for harmonizing a stepwise bass line that ascends or descends through a complete octave. Rather than assigning any harmonic choice to a bass note, the Rule prescribes a specific chord quality and inversion for each scale degree in the bass. In major, the ascending bass receives: I (root position on 1), I6 or V (on 2, depending on context), I6 (on 3), IV or II6 (on 4), V (on 5), VI or I6 (on 6), V6 (on 7), and I (on 8). The descending form adjusts these assignments symmetrically. The result is a fluid, idiomatic harmonization that avoids weak progressions and keeps the upper voices moving smoothly.

Sequences offer a different approach to harmonizing a scalar or sequential bass line. A sequence is a pattern that repeats at successively higher or lower pitch levels. Common diatonic sequences include the descending 5-6 sequence (alternating root-position and first-inversion chords), the ascending 5-6 sequence, and the falling-fifths sequence (root-position chords descending by fifth: I–IV–VII–III–VI–II–V–I). Sequences generate motion and momentum across long stretches of music and are central to development sections, episodes, and transitions.

Both the Rule of the Octave and sequence patterns reflect the galant-era composer's stock of pre-formed solutions to common compositional problems. Rather than reinventing voice leading from scratch for every bass line, trained composers internalized these formulas and deployed them fluently. The Rule connects bass motion to harmonic function in a systematic way; sequences generate harmonic motion through pure pattern repetition. Together they illustrate how 18th-century music is built from layered conventions — melodic (schemas), harmonic (Rule of the Octave), and sequential — that interact to produce the characteristic elegance of the galant style.