The soprano moves from C4 to D4 (up a whole step) while the bass moves from F3 to G3 (up a whole step). Both intervals — C4-F3 and D4-G3 — are perfect fifths. A student claims: 'This is fine because both voices move by the same amount.' Is the student correct?
AYes — moving by the same interval is required for good voice leading
BNo — this is an example of parallel fifths, which is forbidden in strict tonal style
CYes — the rule only applies when the voices move to a unison, not a fifth
DNo — but only because the soprano is in the wrong register
This is exactly the definition of parallel fifths: two voices moving by similar motion and arriving at another perfect fifth. The student has described the situation correctly (same interval maintained) but drawn the wrong conclusion. Moving by the same interval while maintaining a perfect fifth is the problem, not a sign of good technique.
Question 2 Multiple Choice
A student writes a four-part chorale with parallel fifths between the alto and tenor voices. They argue that this is permitted because the rule only forbids parallel fifths between the soprano and bass. Is this correct?
AYes — only outer-voice parallels are forbidden; inner-voice parallels are fine in strict style
BNo — parallel fifths between any voice pair are forbidden in strict tonal style, but outer-voice parallels are most audibly critical and receive the most scrutiny
CNo — parallel fifths are only forbidden when they involve the tenor voice specifically
DYes — Bach himself wrote parallel fifths between inner voices, so the rule does not apply there
In strict four-part style, parallel perfect intervals are prohibited between any pair of voices. However, outer-voice parallels (soprano-bass) are the most perceptually salient because those are the structural anchors the ear follows most closely. Inner-voice parallels are also errors — they're simply less immediately audible. The checking procedure should still cover all voice pairs.
Question 3 True / False
Two voices moving in parallel perfect octaves effectively reduce the perceived number of independent voices in a four-part texture.
TTrue
FFalse
Answer: True
Two voices an octave apart are the same pitch in different registers. In parallel octaves, they reinforce each other at every moment rather than contributing independent melodic content. A four-part texture gains its richness from four genuinely distinct lines; parallel octaves collapse two of those lines into one, effectively reducing the texture to three (or even two) independent voices.
Question 4 True / False
The prohibition on parallel fifths in tonal harmony is an arbitrary stylistic convention with no acoustic basis.
TTrue
FFalse
Answer: False
The rule has a direct acoustic foundation. A perfect fifth has a frequency ratio of 3:2 — one of the simplest and most acoustically pure intervals. This purity causes two voices in a perfect fifth to fuse perceptually into a single tonal entity. When two voices maintain parallel fifths across a motion, they preserve that fusion throughout, and the listener hears one thick line rather than two independent voices. The rule protects voice independence, which is the architectural foundation of the tonal voice-leading tradition.
Question 5 Short Answer
Explain why parallel octaves are considered more problematic than parallel fifths in traditional voice leading.
Think about your answer, then reveal below.
Model answer: A perfect octave is the most extreme case of acoustic fusion: two voices an octave apart are literally the same pitch class, differing only in register. They reinforce each other completely rather than contributing distinct harmonic content. In parallel octaves, those two voices become inseparable for the duration of the passage — the texture is reduced by a full voice. A parallel fifth also causes fusion, but the two pitches remain distinct (a fifth apart), so there is at least some harmonic content from both. The octave is the limiting case where voice independence is entirely lost.
This is why careful composers treat parallel octaves as the more severe violation. Parallel fifths weaken independence; parallel octaves eliminate it. The outer-voice octave is the most egregious form because soprano and bass are the framing voices the ear relies on to define the harmonic progression.