Why Parallel Fifths Sound 'Wrong' — and When to Use Them
The most famous rule in counterpoint has a deep acoustic reason behind it. Here's the physics behind the prohibition — and the cases where great composers broke it on purpose.
Parallel fifths are probably the most repeated rule in any harmony class. "Don't write parallel fifths." But has anyone ever explained to you why?
The Physics Behind the Rule
When two voices move in parallel perfect fifths, their overtone series fuse almost perfectly. The perfect fifth has a frequency ratio of 3:2 — the most consonant interval after the unison and the octave.
The problem is not that they sound bad. The problem is that they sound too good together: they merge into a single acoustic entity and lose their independence as voices. In tonal counterpoint, where each voice must have its own personality, this is exactly what we want to avoid.
"Counterpoint is the illusion of several people speaking at the same time without interrupting each other." — attributed to J.S. Bach
Contrary Motion as the Solution
The alternative that the SATB method proposes is contrary motion: when one voice goes up, the other goes down. This preserves melodic independence and produces more interesting harmonic progressions.
Consider this typical error:
Soprano: C → D
Bass: F → G
Interval: 5th → 5th ← PARALLEL
The solution with contrary motion:
Soprano: C → D
Bass: G → F
Interval: 4th → 5th ← CONTRARY ✓
When the Greats Used Them on Purpose
Shostakovich — the central composer of our method — used parallel fifths with a very specific intention: to evoke emptiness, coldness, or archaism. In the Symphony No. 5 in D minor, the strings in parallel fifths in the third movement create a sense of desolation that is absolutely calculated.
Other historically intentional uses:
- Medieval organum: the primitive liturgical texture was built on parallel fifths. When a Romantic composer used them, they were citing this archaic texture.
- Modal music: in modal contexts (Debussy, Ravel), parallel fifths don't carry the same tonal function and therefore don't "dissolve" voices in the same way.
- Orchestral effects: in string writing, parallel fifths on double stops produce a specific color that can't be obtained any other way.
The Rule as a Starting Point
The prohibition on parallel fifths is not an arbitrary whim of 18th-century theorists. It is the distillation of centuries of choral practice where the intelligibility of each voice was a functional necessity — music was sung in cathedrals, without amplification, and each voice had to be heard clearly.
When you understand why the rule exists, you'll know exactly when breaking it makes musical sense and when it's simply a mistake.
In the Harmony Course, the first lessons work on contrary motion from the very beginning. The Virtual Teacher will flag any parallel fifths in your MIDI exercises and explain exactly which voices are involved.
Have questions about parallel fifths or any other aspect of voice leading? The course has space for your specific questions.