Parallel octaves
Parallel octaves arise when two voices move at the same time in the same direction an octave apart. In harmony teaching they are forbidden, because the two voices cease to be two voices: the octave fuses into a single sound.
- Distance
- octave (ratio 2:1)
- Motion
- parallel, both in the same direction
- In strict writing
- forbidden
- Different in orchestration
- octave doubling is a means of sound
When two voices become one
After the unison the octave is the simplest interval: its frequency ratio is 2:1, and the two notes fuse so strongly that they are felt as the same note in another register. The structure of the note system rests on this. For voice leading a rule follows: if one voice runs in parallel an octave from another, you hear not two voices but one that is doubled. The independence of the voices is lost.
Forbidden: parallel octaves
Two voices run an octave apart, rising by step. You hear only one melody, once low, once high.
Allowed: contrary motion instead of parallel
The same lower voice, but the upper voice goes down. The voices stay recognisable as two lines: they move towards each other.
Hidden octaves: leaping into the octave
Both voices go upwards in the same direction and land on the octave. This is called hidden (or direct) octaves, which are considered unattractive in the outer voices.
In the glossary: Octave · Unison · Parallel fourths
Where they occur anyway
In orchestration, by contrast, octave doubling is a common means: a line becomes stronger and gets colour when flute and violin, or the right and left hands of the piano, play it in octaves. Two-part writing is not wanted there. Parallel octaves can also arise unintentionally, for example when a leading tone is doubled and both leading tones rise to the root. That is why harmony teaching has the rule of not doubling the leading tone, and why the six-three chord calls for attention in doubling.
Parallels that must not be equated
The same reasoning holds for other intervals to differing degrees. Parallel fifths count, like parallel octaves, as forbidden, because the fifth too fuses so strongly. Parallel thirds and sixths, by contrast, are allowed because their notes differ in sound. Parallel unisons are forbidden for the same reason as parallel octaves. Contrary motion is therefore the simplest means of avoiding parallels.
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Related terms
OctaveParallel fourthsParallel seventhsChord inversionSix-three chordUnison
Editor: Maik Krohm · reviewed on 3 October 2026
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