AP® Music Theory · Unit 4 · Topic 4.1
Soprano-Bass Counterpoint
Soprano-bass counterpoint is the relationship between the top and bottom voices of a four-part texture, classified as contrary, similar, parallel, or oblique motion, which controls how independent the two lines sound.
The soprano and bass carry the most musical weight because they're the two voices you actually hear as a shape: the soprano gives the melody, and the bass gives the harmonic foundation everything else is built on. The alto and tenor fill in the middle, but if the outer voices are strong and independent, the texture works even when the inner voices are plain.
Two lines can relate four ways. Contrary motion: they move in opposite directions, one up while the other goes down. Similar motion: both move the same direction but by different amounts. Parallel motion: both move the same direction by the same interval pattern, like two voices sliding up together a third apart. Oblique motion: one voice holds a note while the other moves. Writing a bass line under a given soprano, aim mostly for contrary motion: when the soprano rises, let the bass fall, and vice versa. This naturally keeps the two voices independent and spreads out the harmony instead of letting them shadow each other.
This matters because four-part writing bans parallel fifths and parallel octaves, perfect intervals moving in parallel, because they make two voices collapse into sounding like one line instead of two. Contrary motion dodges this automatically: if the voices are heading opposite directions, they cannot be sliding along in the same parallel perfect interval. That's why soprano-bass counterpoint sets up the voice-leading rules coming next, where you check every pair of voices for forbidden parallels and resolve chords toward cadences.
Key terms
- Contrary motion
- Two voices move in opposite directions, one rising while the other falls.
- Similar motion
- Two voices move in the same direction but by different interval amounts.
- Parallel motion
- Two voices move in the same direction by the same interval, such as both rising a third.
- Oblique motion
- One voice stays on a repeated or held note while the other voice moves.
- Parallel fifths/octaves
- A forbidden pattern where two voices move in parallel through a perfect fifth or octave, erasing their independence.
Watch it explained
- How it worksHow do contrary, similar, parallel, and oblique motion look when two lines are drawn together?
- How it worksWalk me through writing a bass line under a given soprano using mostly contrary motion.
- Worked exampleFind the type of motion when the soprano rises C to D while the bass falls G to F.
- Common mistakeWhy isn't parallel motion between two voices always a mistake?
- Ties togetherHow does contrary motion between soprano and bass avoid the parallel fifths and octaves banned in four-part writing?
Check yourself
End of topic.