In AP Music Theory, similar motion occurs when two voices move in the same direction (both up or both down) but by different intervals, so the distance between them changes. It is one of the four types of relative motion in 18th-century voice leading, alongside parallel, contrary, and oblique motion.
Similar motion is one of the four ways two voices can move relative to each other as a harmonic progression unfolds. Both voices travel in the same direction, but they cover different distances. For example, if the soprano steps up from C to D while the bass leaps up from E to G, that's similar motion. The voices agree on direction but not on interval, so the gap between them changes.
This is exactly where it splits from parallel motion. In parallel motion, the voices move the same direction AND keep the same interval between them (like a chain of parallel thirds). In similar motion, the interval shifts. The CED (PIT-4.A.2) lists similar motion as one of the four motion types you need to recognize, and PIT-4.A.1 explains why it matters. Voice leading in the common-practice style is all about smooth lines and independent voices. Similar motion is allowed and common, but because both voices move the same way, it creates less independence than contrary or oblique motion, so 18th-century style uses it with some care, especially when the voices land on a perfect fifth or octave.
Similar motion lives in Topic 4.1, Harmony and Voice Leading I (Unit 4), and it directly supports learning objective 4.1.A: identifying and applying 18th-century voice-leading procedures through score analysis, error detection, writing exercises, and contextual listening. You can't do any of those tasks without instantly classifying motion between voices. It also feeds 4.1.B and 4.1.E, where you compose a bass line under a given soprano. As you write each bass note, you're constantly choosing between parallel, similar, contrary, and oblique motion, and those choices determine whether your part writing earns points or racks up errors. A line that leans too hard on similar motion in the outer voices is how forbidden parallels sneak in, since two voices already moving the same direction are one careless interval away from parallel fifths or octaves.
Keep studying AP Music Theory Unit 4
Visual cheatsheet
view galleryParallel motion (Unit 4)
Parallel motion is similar motion's stricter sibling. Both move voices in the same direction, but parallel keeps the interval identical the whole time. Think of parallel motion as a special case of same-direction movement, and similar motion as the looser, more common version.
Contrary motion (Unit 4)
Contrary motion is the opposite of similar motion. Voices move in opposite directions, which maximizes independence between the lines. When you compose a bass line under a soprano, contrary motion is often your safest move because it can never produce parallel fifths or octaves.
Voice leading (Unit 4)
Similar motion is one of the four building blocks of voice leading, the bigger system that governs how chords connect smoothly (PIT-4.A.1). Knowing the motion types is step one; the real skill is balancing them so each voice sounds like its own melody.
Consecutive thirds and sixths (Unit 4)
When voices move in the same direction by the same interval and land on thirds or sixths, that's parallel motion done right. The contrast helps you see what makes similar motion different. Consecutive thirds keep the interval fixed, while similar motion lets the gap stretch or shrink.
Similar motion shows up two ways. In multiple choice, expect motion-classification questions where you look at two voices in a score excerpt and name the motion type, or error-detection items where you spot voice-leading problems caused by careless same-direction movement. In the free-response section, the part-writing tasks make you apply it. The 2025 SAQ Q7, for example, asks you to complete a bass line under a given melody following 18th-century voice-leading procedures, then label the harmony with Roman and Arabic numerals. Graders check the motion between your bass and the soprano, so writing all your moves in similar motion with the melody is a fast way to create parallel fifths or octaves and lose points. The practical skill is this: classify the motion between any two voices on sight, and mix in contrary and oblique motion when you compose so your lines stay independent.
Both terms mean the voices move in the same direction, which is why they get mixed up. The difference is the interval between the voices. In parallel motion, the interval stays exactly the same (C-E moving to D-F is parallel thirds). In similar motion, the interval changes (C-E moving to D-G is similar motion, since a third became a fourth). The distinction matters for grading: parallel fifths and octaves are forbidden in 18th-century style, while similar motion into most intervals is perfectly fine.
Similar motion means two voices move in the same direction but by different intervals, so the distance between them changes.
It is one of the four motion types in the CED (PIT-4.A.2), along with parallel, contrary, and oblique motion.
The difference from parallel motion is the interval: parallel keeps the same interval between voices, similar does not.
Similar motion is allowed in 18th-century voice leading, but overusing it in the outer voices is the easiest way to stumble into forbidden parallel fifths and octaves.
On part-writing FRQs like a bass line added to a given soprano, mixing similar motion with contrary and oblique motion keeps your voices independent and protects your score.
Similar motion is when two voices move in the same direction (both ascending or both descending) but by different intervals, so the space between them changes. It's one of the four motion types tested in Unit 4, Topic 4.1.
No. In parallel motion the voices move the same direction and keep the exact same interval between them, like a chain of parallel thirds. In similar motion the voices move the same direction but the interval changes, like a third expanding to a fifth.
Yes, similar motion is common and stylistically normal. The danger zone is moving two voices in similar motion into a perfect fifth or octave, which is where forbidden parallels tend to come from, so the style balances it with contrary and oblique motion to keep voices independent.
They're opposites in direction. Similar motion sends both voices the same way (both up or both down), while contrary motion sends them in opposite directions. Contrary motion creates the most independence between voices, which is why it's often preferred between soprano and bass.
You'll classify motion types in multiple-choice score analysis and apply them in part-writing FRQs. The 2025 SAQ Q7, for instance, required composing a bass line under a melody using 18th-century voice-leading procedures, where your motion choices against the soprano are part of what's graded.