Parallel fifths · Rules
Two voices a perfect fifth apart do not move to another perfect fifth in the next chord, whether by similar or contrary motion, nor as compound intervals (twelfth). Between soprano and bass, a diminished fifth does not move to a perfect fifth either. A perfect fifth may move to a diminished one.
- Parallel fifths: Two voices a perfect fifth apart do not move to another perfect fifth in the next chord, whether by similar or contrary motion, nor as compound intervals (twelfth). Between soprano and bass, a diminished fifth does not move to a perfect fifth either. A perfect fifth may move to a diminished one. (Piston, ch. 3, pp. 29–30 and 34)