"although Pythagorean theory, which provided the principal intellectual framework for understanding music during the early Middle Ages, seemed to recommend uniform consonances such as fourths, fifth and octaves, musicians discovered that voices spaced in these intervals and moving rigidly in parallel tend be heard as one. In fact, whenever two voices come together in a fifth or an octave — or worst of all, a unison — there is a danger that they will fuse like two converging rivers, so that we lose track of one or the other. [...]
This came to be seen as a guiding rule of composition that was to be observed even when composers were not writing counterpoint. And so we very rarely find chord sequences in the Classical and Romantic eras that involve simultaneous, identical movements of all the notes.* There was actually no good reason for this prohibition, however, when the intention was not to maintain separate voices, although it wasn’t until the late nineteenth century that composers such as Claude Debussy and Maurice Ravel dared to ignore it.
*It was considered particularly important to avoid parallel movement of fifth and octaves, since these create the greatest danger of fusion. Parallel thirds and sixths are much more common, because these are less consonant, and so we’re less likely to interpret them as overtones."
- Philip Ball, The Music Instinct
From Bach fugues to Indonesian gamelan, from nursery rhymes to rock, music has cast its light into every corner of human culture. But why music excites such deep passions, and how we make sense of musical sound at all, are questions that have until recently remained unanswered. Now in The Music...
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[Edit: of course, there are also other ways to prevent voices from fusing---giving each one to an instrument with a different timbre, panning or spatialization, making some voices slightly sharp or slightly flat (but not so much they sound noticeably out of tune), etc.]
