Counterpoint
Parallel Fifths and Octaves: Why the Rule Exists
Parallel fifths and octaves are voices that move together and keep a perfect interval. Here is why strict voice leading forbids them and where the rule bends.
Perfect interval
A unison, fourth, fifth or octave, which has one standard size and becomes augmented or diminished when widened or narrowed.
Also called: perfect consonance
A perfect interval is a unison, fourth, fifth or octave. These intervals have one standard size, called perfect, and no major or minor forms. Lowering a perfect interval by a semitone makes it diminished, and raising it makes it augmented. A perfect fifth, C to G, has seven semitones. A diminished fifth, B to F, has six, and an augmented fifth, C to G♯, has eight. The perfect fifth forms the outline of most triads, and perfect intervals are among the most stable in tonal harmony. In traditional four-part writing, parallel perfect fifths and octaves between voices are avoided because they weaken the independence of the lines.
Related terms
- Major intervalA second, third, sixth or seventh that spans the number of semitones found in the major scale on its lower note.
- Minor intervalA second, third, sixth or seventh one semitone smaller than the major interval with the same letter names.
- Augmented intervalAn interval one semitone wider than a major or perfect interval with the same letter names, such as F to B.
- Diminished intervalAn interval one semitone narrower than a perfect or minor interval with the same letter names, such as B to F.