Parallel octaves
Parallel octaves between two voices, forbidden in strict part-writing.
What it means
Parallel octaves arise when two voices move in succession in the same direction at the distance of an octave, so that the octave interval between them is retained. In strict four-part writing and in counterpoint they count as an error. The reason lies in the independence of the voices: two voices an octave apart sound almost like a single one, so that the voice-leading loses its independence.
Musical use
The prohibition applies in chorale writing and in the theory of part-writing since the Renaissance and is part of the classical rules of voice-leading, as is the prohibition of parallel fifths. For hidden or leaping parallels the rule has been applied more leniently. Composers, however, often broke the rule deliberately when they wanted to reinforce a melody.
Parallel octaves should be distinguished from deliberate octave doubling, which is used in the orchestra or the organ to reinforce sound and does not violate voice-leading in the sense of part-writing theory, and from parallel thirds and sixths, which are allowed as pleasing.
Performance practice
When you write or realize a texture, check every pair of voices for parallel octaves: make sure two voices do not move in the same direction towards an octave. Lead one of the voices in contrary motion or choose a different chord note.