Questions: Voice Motion Types: Parallel, Contrary, and Oblique

Question 1

In a four-voice chorale, the soprano and alto both move up by a third from one chord to the next. What type of motion is this, and is it a voice-leading error?

• A. Parallel motion — and it is a voice-leading error because parallel motion is forbidden
• B. Parallel motion — and it is acceptable because parallel thirds and sixths are the foundation of harmonic texture
• C. Oblique motion — the voices are sustained, not truly moving
• D. Contrary motion — moving in the same direction counts as contrary when the interval is maintained

Answer: (B)

Moving in the same direction by the same interval is parallel motion. However, parallel thirds are not only allowed but encouraged in traditional part writing — they reinforce harmonic texture without collapsing voice independence. Only parallel perfect intervals (fifths and octaves) are forbidden because they cause two voices to merge acoustically into a single strand. The misconception that all parallel motion is wrong confuses a rule about perfect intervals with a blanket prohibition on parallel motion.

Question 2

Why is contrary motion between soprano and bass especially recommended by voice-leading pedagogy?

• A. Contrary motion guarantees that no parallel thirds or sixths can occur between soprano and bass
• B. Contrary motion between outer voices structurally prevents parallel fifths and octaves from occurring between them, while also creating textural openness
• C. Contrary motion is only preferred because it makes the soprano line more singable
• D. The outer voices must always move in contrary motion — it is an absolute rule with no exceptions

Answer: (B)

When soprano and bass move in opposite directions, they cannot arrive at the same perfect interval they started on — if they were a fifth apart moving in contrary motion, they cannot still be a fifth apart afterward. This geometric fact structurally prevents the most dangerous voice-leading errors. Simultaneously, contrary outer-voice motion creates the breathing quality of opening and closing textures. Option C is too narrow (it's about structural protection, not just singability), and option D overstates it — contrary motion is a strong preference, not an absolute rule.

Question 3

Parallel motion between two voices usually creates a voice-leading error in traditional four-part harmony.

• T. True
• F. False

Answer: False

Only parallel perfect intervals — parallel fifths and parallel octaves — are errors. Parallel thirds and parallel sixths are not only permitted but are the harmonic backbone of tonal part writing. When soprano and alto both move up a third in the same direction by the same interval, the voices retain their independence because the interval is imperfect and doesn't cause acoustic merging. The rule against parallel motion is actually a rule against parallel *perfect* intervals specifically.

Question 4

Oblique motion — where one voice holds a common tone and another moves — tends to create the smoothest possible connection between two chords at that voice.

• T. True
• F. False

Answer: True

A common tone held across a chord change is oblique motion taken to its logical extreme: one voice makes no movement at all. Because the goal of voice leading is smooth, connected motion, zero motion is the smoothest possible outcome. Common tones between adjacent chords are highly prized precisely because they provide oblique motion 'for free' — the voice simply sustains, contributing stability and connection without any melodic activity or risk of error.

Question 5

Explain why contrary motion between the soprano and bass prevents parallel perfect fifths from occurring between them, while parallel motion does not provide this guarantee.

Model answer: If soprano and bass are a fifth apart and move in contrary motion, one moves up while the other moves down. For them to still be a fifth apart after the move, the interval between them must expand and then contract by exactly the same amount — geometrically impossible when they're moving in opposite directions by general stepwise motion. Parallel motion, by contrast, keeps the voices moving together, so if they start a fifth apart and both move up by the same interval, they end a fifth apart again. Contrary motion makes the recurrence of a parallel perfect interval structurally unlikely.

The logic is geometric: contrary motion diverges or converges the voices, making it very hard to land on the same perfect interval twice consecutively. Parallel motion maintains constant intervals, which is exactly the condition that produces parallel fifths. This is why contrary outer-voice motion is the structural antidote to the most common voice-leading errors — it works by the geometry of interval change, not by stylistic convention alone.