parallel fifths
The movement of two voices in perfect fifths in the same direction, prohibited in traditional part writing
In Depth
Parallel perfect fifths occur when two voices maintain a perfect fifth interval while moving in the same direction. This is forbidden in Classical part writing because the hollow sound of open fifths was considered to undermine the independence of the voices. Medieval organum, by contrast, was built entirely on parallel fifths. In modern music, parallel fifths are used freely — Debussy, Ravel, and virtually all popular music employ them without concern. The prohibition remains a cornerstone of academic harmony pedagogy.
Debussy deliberately wrote parallel fifths as a rejection of conservatory rules, telling his teacher that the rules were made by people who could not write good music.