Voice Leading Overview
The Independence of the Voices
In the previous section:
- The two parts move up and down independently of each other.
- This mixture of contrary and similar motion allows the lines to have melodies that move independently of each other and facilitates imitation.
In 16th century polyphony (and in most tonal music) parallel 3rds, 6ths and 10ths are considered to make good voice leading provided that not too many successive similar intervals are deployed in sequence.
It is difficult to explain why parallel 3rds, 6th and 10ths should be considered OK whereas parallel unisons, 5ths, octaves and 12ths should not. It's partly to do with the purity of the latter perfect intervals and partly due to the fact that when sequences of thirds and 6ths follow each other they can switch between major and minor versions of the same intervals, so that some degree of variety is maintained whereas parallel perfect intervals always produce exact parallel movement with no such variation and so destroy the independence of the parts. This rule about not allowing parallel 5ths, 8ves unisons and 12ths is the most important and consistently applied "rule" in all of voice leading and is also generally adhered to in most 16th century and tonal music.
Note: Where large numbers of instruments are involved, parallel octaves and unisons are used where voices follow the same melodic line for some length of time (hence no counterpoint intended). This is never true of parallel 5ths or 12ths in tonal music. Parallel 4ths can be used between upper voices but only when accompanied by parallel 3rds and 6ths.
See also the section Movement in 10ths
for further details about the voice leading possibilities of movement
in parallel 10ths.
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