In AP Music Theory, parallel motion is one of the four types of linear movement between two voices, where both voices move in the same direction by the same interval. Parallel thirds and sixths are fine, but parallel perfect fifths and octaves break 18th-century voice-leading rules.
Parallel motion happens when two voices move in the same direction by the same interval. If the soprano goes up a third and the alto also goes up a third, keeping the same interval between them, that's parallel motion. The CED lists it as one of four ways two voices can move relative to each other (PIT-4.A.2), alongside similar, oblique, and contrary motion.
Here's the part the exam actually cares about. Parallel motion itself is not an error. Parallel thirds and parallel sixths sound great and show up constantly in chorales. The problem is parallel motion at perfect intervals. When two voices move in parallel perfect fifths or perfect octaves, they stop sounding like two independent voices and start sounding like one thickened voice. That kills the independence of voices that 18th-century style demands, which is why parallel fifths and octaves are the classic part-writing errors graders hunt for.
Parallel motion lives in Topic 4.1 (Harmony and Voice Leading I) under LO 4.1.A, where you identify and apply 18th-century voice-leading procedures through score analysis, error detection, part writing, and contextual listening. The essential knowledge (PIT-4.A.1 and PIT-4.A.2) makes the goal explicit. Voice leading should achieve linear smoothness and independence of voices, and parallel motion is the motion type most likely to threaten that independence when it happens at perfect intervals.
It also matters in Topic 5.6 (Cadential 6/4 Chords). Under LO 5.6.B, the sixth and fourth above the bass in a cadential 6/4 must resolve down by step into the dominant (PIT-4.E.1). Those upper voices often move in parallel motion with each other, descending by step at the same time. That's parallel motion done right, and knowing the difference between good and bad parallels is exactly what error-detection questions test.
Keep studying AP Music Theory Unit 4
Visual cheatsheet
view galleryParallel Fifths (Unit 4)
Parallel fifths are the specific forbidden case of parallel motion. Two voices a perfect fifth apart moving in the same direction to another perfect fifth is the single most common error you'll be asked to spot in error-detection questions.
Voice Leading (Unit 4)
Parallel motion is one of the four motion types that voice leading is built from. The whole 18th-century style is a balancing act, using enough parallel thirds and sixths for smoothness while avoiding parallel perfect intervals to keep voices independent.
Contrary Motion (Unit 4)
Contrary motion is the opposite end of the spectrum. Voices move in opposite directions, which maximizes independence. When you write a bass line against a given soprano (LO 4.1.B), favoring contrary motion is the easiest way to dodge accidental parallels.
Cadential 6/4 Chords (Unit 5)
The cadential 6/4 resolution is parallel motion working in your favor. The sixth and fourth above the bass both step down to the fifth and third of the dominant, often moving in parallel, while the bass holds. The figures 6/4 to 5/3 literally spell out that stepwise descent.
Multiple-choice questions ask you to classify the motion between two voices (a practice question puts it plainly: in parallel motion, how do two voices move?), or to explain how voice-leading rules from the Common Practice Period create independence of voices. On the part-writing FRQs, parallel motion is a scoring issue rather than a vocabulary word. When you realize a figured bass or harmonize a melody in four voices, writing parallel perfect fifths or octaves between any pair of voices costs you points. Error-detection questions flip it around and hand you a flawed passage, expecting you to circle the parallel fifths or octaves. The safe habit is to check every adjacent chord pair for perfect intervals moving in the same direction, especially between the bass and another voice.
Parallel motion is a neutral category, just two voices moving the same direction by the same interval. Parallel fifths are one illegal subset of it. Parallel thirds and sixths are stylistically encouraged; parallel perfect fifths and octaves are forbidden because they collapse two voices into one sound. If you say 'parallel motion is wrong' on the exam, you've overgeneralized. Only the perfect-interval parallels are errors.
Parallel motion means two voices move in the same direction by the same interval, and it's one of the four motion types in the CED (parallel, similar, oblique, contrary).
Parallel motion itself is legal; parallel perfect fifths and perfect octaves are the errors, because they destroy the independence of voices that 18th-century style requires.
Parallel thirds and sixths are common and stylistically good in chorale writing, so don't avoid all parallels, just the perfect ones.
In a cadential 6/4, the sixth and fourth above the bass resolve down by step, often in parallel motion, which is the textbook example of parallel motion used correctly.
On part-writing FRQs, check every pair of voices between consecutive chords for perfect fifths or octaves moving in the same direction before you move on.
Parallel motion is when two voices move in the same direction by the same interval, like soprano and alto both stepping up while staying a third apart. The CED lists it as one of four types of linear motion between voices, along with similar, oblique, and contrary motion.
No. Parallel thirds and sixths are perfectly fine and very common in chorale style. Only parallel perfect fifths and parallel octaves are voice-leading errors, because they make two voices sound like one and wreck voice independence.
In parallel motion, both voices move the same direction AND keep the same interval between them. In similar motion, they move the same direction but the interval changes, like one voice stepping while the other leaps. The exam can ask you to label motion between voices, so the interval check is what separates the two.
Perfect fifths and octaves blend so completely that two voices moving in parallel at those intervals stop sounding independent. Since 18th-century voice leading (PIT-4.A.1) is built on the independence of voices, parallels at perfect intervals defeat the whole point of writing four separate parts.
Often, yes. The sixth and fourth above the bass both resolve down by step into the dominant chord (PIT-4.E.1), and those two upper voices frequently descend together in parallel thirds while the bass stays put. It's parallel motion the style approves of.
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