Parallel fifths
Forbidden fifth movements in strict writing.
What it means
One speaks of parallel fifths when two voices forming a perfect fifth move in the same direction and thereby reach a perfect fifth again. In strict writing this counts as an error. The same holds for parallel octaves and unisons.
Musical use
The reason for the prohibition is audible and not mere convention. The fifth is a perfect consonance and merges strongly: two voices led in parallel fifths are not perceived as two lines but as one with a doubling. This loses exactly what polyphonic writing is about — the independence of the voices. Parallel thirds and sixths are allowed for the same reason: they merge less, and both voices remain audible.
Some distinctions belong to the rule. Hidden or direct fifths occur when a fifth is approached by leap in the same direction; they are treated with varying strictness depending on the teaching tradition, more strictly in the outer voices than between middle voices. A perfect fifth following a diminished one does not count as parallel, because the interval changes. And horn fifths count as a permissible special case, because an intermediate step lies between them.
Historically the prohibition is limited. Early organum rests on parallel fifths; in nineteenth- and twentieth-century music they were deliberately used as a tonal device, and in the power chord of popular music they are the load-bearing element.
The prohibition holds because two voices running in fifths merge tonally into one and lose their independence. In the Middle Ages, by contrast, parallel fifths were usual in organum. Hidden fifths, in which two voices run by leap in the same direction into a fifth, are likewise to be avoided in the outer voices. In rock music, parallel fifths as power chords form a foundation.
Performance practice
When writing your own part-writing, watch above all the outer voices — there parallels stand out most clearly. When in doubt, contrary motion helps. And when playing: where voices are led in parallel, shape them as one sound with the same articulation, instead of bringing out both individually.