Parallel motion
Voices moving in the same interval.
What it means
Parallel motion means that two voices move in the same direction while keeping the same interval. It is one of the four types of motion in part-writing theory, alongside contrary motion, oblique motion with one voice held, and similar motion in the same direction with a changing interval.
Musical use
The judgement depends on the interval. Parallel thirds and sixths count as pleasant-sounding and are usual in composition: both are imperfect consonances, do not merge completely, and the voices remain audible as two. Parallel fifths and octaves, by contrast, are forbidden in strict writing, and for an audible reason: they merge so strongly that two voices become one with a doubling — independence is lost. The same holds for the unison and the octave.
This rule has grown historically and is not universally valid. The early polyphony of organum rested precisely on parallel fifths and fourths; the fifteenth-century faux-bourdon leads voices in parallel sixths; horn fifths count as a permissible special case. And in the music of the nineteenth and twentieth centuries, parallel leading was deliberately used as a tonal device, where not the independence of the voices but tone colour was the goal.
In ensemble playing, parallel motion is also a practical matter: voices led in parallel must agree in articulation, dynamics, and intonation, so that they work as one sound.
Part-writing theory distinguishes parallel motion, contrary motion, oblique motion, and contrary motion with a sustained tone. Parallel thirds and sixths are allowed in strict writing and pleasant-sounding, parallel fifths and octaves forbidden. In Impressionism, whole chords are shifted in parallel, a technique called planing.
Performance practice
Play voices led in parallel as one: same length, same attack, same dynamics — every deviation stands out immediately. Watch the intonation, especially with thirds, which want to be taken rather low. And listen, in polyphonic writing, for where the voices are meant to become independent; there the paths separate, and this wants to be audible.