Related
"Parallel fifths and octaves are forbidden" is the first rule most students hear, and the most misunderstood — because it is not about sounding good or bad.
The real reason
Perfect fifths and perfect octaves fuse too well (see perfect intervals). If two voices keep a fifth or octave between them from one chord to the next, they stop being two lines and become one line with extra thickness.
So what is avoided is not "these two notes" but "in succession". The consequence is not ugliness — it is that the texture has one voice fewer than it appears to have.
That runs directly against the goal of voice leading, whose whole effort is to keep each line audible.
| Consecutive interval | Consequence |
|---|---|
| parallel fifths | two voices merge into one (the fifth's hollowness makes it obvious) |
| parallel octaves | two voices merge, even more completely — it sounds like one thickened voice |
| parallel thirds and sixths | permitted — imperfect consonances do not fuse |
That last row is the key comparison: the rule forbids parallel *perfect* consonances, not parallel motion.
Two related cases
| Case | Description | Treatment |
|---|---|---|
| hidden (direct) fifths or octaves | two voices arrive at a fifth or octave by similar motion with a leap in the upper voice | also avoided — a similar merging is heard |
| similar motion with a step in the upper voice | far weaker effect | generally permitted |
Why the rule gets exaggerated
Because it is the easiest to teach: simple to spot and simple to mark. It therefore comes to stand for harmony in general, and is even misreported as "parallel fifths sound bad".
The reality: parallel fifths are common in folk music, plainchant, Impressionism and rock, and they do not grate. They are banned in four-part writing exercises because those exercises aim at voice independence, not at pleasantness.
Diagram: fusion versus independence
Listen: two lines or one
Use the progression player on a standard progression. Notice how, when bass and upper voices move in different directions, each line remains separately audible — exactly what parallel fifths give up.
Common misconceptions
- "Parallel fifths sound bad." They do not; folk and rock are full of them. They are avoided for the sake of voice independence.
- "Parallel thirds and sixths are also banned." They are permitted. Imperfect consonances do not fuse two lines into one.
- "The rule no longer applies." Impressionism, jazz and rock use them freely; but as long as the texture is four-part writing meant to keep the voices independent, the rule applies.
- "Arriving at a fifth by similar motion is always wrong." It depends on the upper voice: hidden fifths and octaves with a leap are normally avoided, while arriving by step is much freer.
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Listen in the library
All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).
Sources
- 平行五度与平行八度在和声写作中被避免，原因在于声部独立性的丧失，属对位与和声学通则
- 隐伏五八度（同向到达）同样受限、平行三六度则允许，为通行和声写作表述
Used for fact-checking only; all prose is original.