Parallel octaves

In music, consecutive fifths, or parallel fifths, are progressions in which the interval of a perfect fifth is followed by a different perfect fifth between the same two musical parts (or voices): for example, from C to D in one part along with G to A in a higher part. Octave displacement is irrelevant to this aspect of musical grammar; for example, parallel twelfths (i.e. an octave plus a fifth) are equivalent to parallel fifths.

Though used in, and evocative of, various kinds of popular, folk, and medieval music, parallel motion in perfect consonances (P1, P5, P8) is strictly forbidden in species counterpoint instruction (1725–present) and during the common practice period, consecutive fifths were strongly discouraged. This was primarily due to the notion of voice leading in tonal music, in which, "one of the basic goals...is to maintain the relative independence of the individual parts." The perfect intervals (the 4th, 5th and octave) sound 'pure'. This has the psychoacoustic effect momentarily of 'fusing' the separate voices and weakening their harmonic independence. Thus voices moving in parallel perfect intervals lose their harmonic autonomy momentarily. An analogy is to consider the flight of birds ‘welded’ together.

A common theory is that the presence of the 3rd harmonic of the harmonic series influenced the creation of the prohibition.

Singing in consecutive fifths may have originated from the accidental singing of a chant a perfect fifth above (or a perfect fourth below) the proper pitch. Whatever its origin, singing in parallel fifths became commonplace in early organum and conductus styles. Around 1300, Johannes de Garlandia became the first theorist to prohibit the practice. However, parallel fifths were still common in 14th-century music. The early 15th-century composer Leonel Power likewise forbade the motion of "2 acordis perfite of one kynde, as 2 unisouns, 2 5ths, 2 8ths, 2 12ths, 2 15ths," and it is with the transition to Renaissance-style counterpoint that the use of parallel perfect consonances was consistently avoided in practice. The convention dates approximately from 1450. Composers avoided writing consecutive fifths between two independent parts, such as tenor and bass lines.

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