Questions: Voice Leading: Avoiding Parallel Fifths and Octaves

Question 1

In four-part writing, the soprano moves from C5 to D5 while the bass moves from G3 to A3 simultaneously. Is this a voice-leading violation?

• A. No — the voices are moving in different octaves, so the interval doesn't apply
• B. No — only inner voices are subject to the parallel fifths rule
• C. Yes — C-G and D-A are both perfect fifths, and moving through them in parallel reduces harmonic independence
• D. Yes — any parallel motion in contrary registers is prohibited

Answer: (C)

C to G is a perfect fifth, and D to A is also a perfect fifth. When two voices move in parallel motion through two consecutive perfect fifths, this is a parallel fifths violation — regardless of which octave the voices occupy. The prohibition applies to all six pairings in four-part writing (S-A, S-T, S-B, A-T, A-B, T-B), not just inner voices. The acoustic reason is that these voices begin to fuse rather than sound independent.

Question 2

A student argues: 'Parallel thirds and parallel fifths are essentially the same — both are just two voices moving in the same direction by the same interval size, so they should have the same effect.' How would you evaluate this argument?

• A. Correct — parallel motion of any interval tends to weaken voice independence equally
• B. Incorrect — parallel thirds blend warmly without fusing voices, while parallel fifths cause acoustic fusion that destroys independence; the rule is specific to perfect intervals
• C. Incorrect — parallel thirds are actually more problematic because thirds are closer together
• D. Correct — both are avoided in strict counterpoint for the same acoustic reason

Answer: (B)

The student's argument confuses parallel motion (same direction) with acoustically identical effects. Parallel thirds (ratio approximately 5:4) and sixths blend pleasantly but do not fuse — the two voices remain perceptually distinct. Parallel perfect fifths (ratio 3:2) and octaves (2:1) cause the voices to merge acoustically, reducing four-part texture to fewer effective voices. The rule targets perfect consonances specifically because of their strong overtone relationships, not all parallel motion.

Question 3

Parallel octaves between the soprano and bass are a voice-leading error, but parallel octaves between inner voices (alto and tenor) are generally acceptable in four-part writing.

• T. True
• F. False

Answer: False

Parallel octaves (and parallel perfect fifths) are prohibited between any pair of voices in four-part writing — not just the outer voices. All six pairings (S-A, S-T, S-B, A-T, A-B, T-B) must be checked. The soprano-bass pair is most audible and most commonly cited, which may create the impression the rule is limited to outer voices, but inner-voice parallel octaves also collapse two voices into one and are equally forbidden in strict counterpoint.

Question 4

The acoustic reason parallel perfect fifths reduce harmonic independence is that the 3:2 frequency ratio of a perfect fifth causes two voices moving in parallel through such intervals to fuse perceptually rather than remain distinct.

• T. True
• F. False

Answer: True

This is the underlying acoustic principle. Perfect fifths and octaves are the strongest consonant intervals in the overtone series. When two voices maintain a perfect fifth or octave while moving in parallel, the overtone relationship between them is so strong that the ear begins to merge them into a single, distinctive sound — a hollow medieval drone quality. This acoustic fusion is what reduces the harmonic independence that four-part writing depends on.

Question 5

Why are parallel thirds acceptable in voice leading when parallel perfect fifths are not? What acoustic property distinguishes them?

Model answer: Parallel thirds blend warmly but do not cause acoustic fusion — the two voices remain perceptually distinct because thirds (frequency ratio approximately 5:4) do not have the same overtone-series dominance as perfect fifths (3:2) or octaves (2:1). Perfect fifths and octaves are the intervals most strongly reinforced by the natural overtone series, so when two voices move in parallel through them, the ear merges them. Thirds and sixths lie further from this perfect alignment; they blend without fusing, allowing each voice to retain its identity.

The rule is not about avoiding parallel motion in general — parallel thirds and sixths are actually desirable for smooth harmonization. The rule is specifically about intervals that cause acoustic fusion: perfect consonances with the simplest frequency ratios. Understanding the acoustic basis explains why parallel thirds are a standard technique (used throughout folk and classical music to harmonize melodies) while parallel fifths hollow out the texture.