AP® Music Theory · Unit 4 · Topic 4.1
Parallel Fifths and Parallel Octaves
Parallel fifths and octaves happen when two voices keep the same fifth or octave interval while moving in the same direction, collapsing their independence into one fused sound. Voice leading forbids this motion itself, no matter how legal the chords on either side are.
Four-voice writing works because each voice sounds like its own line, not a shadow of another. A perfect fifth or octave is so consonant that two voices sitting in that interval start to blend into a single sound. If both voices then slide in the same direction and land on another perfect fifth or octave, the ear hears one fat voice doubling itself instead of two independent lines. That is the whole complaint: it is not about the chords being wrong, it is about the motion erasing a voice.
Spotting this means tracking one pair of voices at a time, say tenor and bass, across a chord change. Find the interval between them in the first chord. If it is a perfect fifth or octave, check their direction into the next chord. Same direction plus landing on another perfect fifth or octave, in either order, is the violation, even if the fifths are not identical in size, like a diminished fifth sliding into a perfect fifth, because the rule cares about the perfect interval at the finish line and the shared direction getting there, not about the exact size of the first interval. A legal, correctly spelled destination chord does not excuse it, since the rule judges the motion between the chords, not the chord itself.
This sits right before cadences and full progressions because every progression is built from one chord move to the next, and each move gets checked for parallels before anything else. Once a progression is clean of parallels, the same voices get judged on how smoothly they approach a cadence, so this rule is the first filter every later skill in the unit has to pass through.
Key terms
- Parallel motion
- Two voices moving in the same direction by the same or different amounts, keeping a similar melodic shape.
- Perfect fifth
- An interval of seven half steps, consonant enough that two voices in it can sound fused rather than separate.
- Perfect octave
- An interval of twelve half steps where the two pitches share the same letter name, sounding almost like one doubled pitch.
- Contrary motion
- Two voices moving in opposite directions, which widens or narrows the interval between them instead of preserving it.
- Voice independence
- The property of each voice in a texture sounding like its own distinct melodic line rather than a doubled copy of another voice.
Watch it explained
- How it worksHow do two voices a fifth apart moving the same direction stop sounding like two voices?
- How it worksWalk me through spotting parallel octaves between tenor and bass across a chord change.
- Worked exampleFind the parallel fifths hiding when C major moves to D minor with every voice stepping up.
- Common mistakeWhy isn't it fine to move two voices in parallel fifths as long as the destination chord is perfectly legal?
- Common mistakeWhy does moving from a diminished fifth to a perfect fifth break the same rule as moving between two perfect fifths?
Check yourself
End of topic.