Parallel octaves

Two voices may not move to an octave from an octave in the same direction: consecutive octaves. This is distinct from doubling — a line doubled in octaves throughout is one voice reinforced, a legitimate scoring choice — the fault is two hitherto independent voices arriving at octaves in succession, which collapses them into one at the moment of motion. The unison is governed the same way, and more strictly still, since it erases the second voice entirely.

Most part-writing accidents of this kind happen at cadences and in outer voices, where soprano and bass both want to move to the tonic — which is why the cadential formulas assign the two voices different routes there. The distinction from doubling matters daily: reinforcing a bass line at the octave is orchestration, but letting the alto shadow the soprano into octaves for two chords is a lost voice, and the ear registers the difference immediately.

• Writing octaves between bass and soprano at the cadence by sending both to the tonic in the same direction.
• Confusing deliberate octave doubling of one line with two independent voices falling into octaves, and excusing the second as the first.
• Missing unison cases, where two voices land on the same pitch in motion and one line simply vanishes.
• Letting octaves stand between a voice and an inner pedal or repeated tone on the theory that a static voice cannot offend.
• Producing octaves by ornament — a passing tone landing the pair on the octave a beat late does not launder them.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, the four motions and the three rules — The octave case of the same exclusion: perfect to perfect in similar motion.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), The preliminary rules (consecutive octaves) — The ban restated with the unison included.