How do the outer voices, soprano and bass, move against each other to make good counterpoint?
Topic 4.1 Soprano-Bass Counterpoint: write and identify the four kinds of motion between the outer voices and avoid parallel perfect intervals.
A focused answer to AP Music Theory Topic 4.1, covering the four types of motion (parallel, similar, contrary, oblique) between the outer voices, the ban on parallel perfect fifths and octaves, the preference for contrary motion, and starting and ending intervals, with a worked example.
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What this topic is asking
The College Board (Topic 4.1) wants you to control the relationship between the two outer voices, soprano and bass, by recognizing and writing the four types of motion between them and by avoiding parallel perfect fifths and octaves. The outer-voice framework is the skeleton onto which the inner voices are later added.
The four types of motion
These four categories describe every possible pair of moves between two voices. Contrary and oblique motion keep the voices sounding independent and almost never cause forbidden parallels, so they are favored; parallel and similar motion need care because they can slide into parallel perfect intervals.
The ban on parallel perfect fifths and octaves
The reason is acoustic and stylistic: perfect fifths and octaves blend so completely that moving in parallel makes two voices sound like one reinforced line, which contradicts the goal of four independent parts. The same logic bans parallel unisons. Hidden (or direct) fifths and octaves, where the outer voices reach a perfect interval by similar motion with a leap in the soprano, are also avoided at cadences.
Why the outer voices come first
The central idea is that the soprano and bass are the most exposed voices, so getting their counterpoint right is the foundation of correct part-writing. A listener hears the top line as the melody and the bottom line as the harmonic anchor, and parallels between them are the most audible and the most penalized. By sketching a strong outer-voice frame first, using mostly contrary and oblique motion and saving leaps for the bass, you guarantee the harmonic skeleton is sound before you fill in the alto and tenor. This top-and-bottom-first approach is why the course teaches two-voice counterpoint before full SATB writing: if the outer pair is clean, the inner voices mostly follow by the shortest smooth route, and you avoid burying an error where it is hard to find.
Building good outer-voice motion
To write the outer voices, choose a bass line that supports the harmony, then set a soprano that moves mostly by step and in contrary or oblique motion to the bass. After each pair of notes, check the harmonic interval: if you have two perfect fifths or two octaves in a row, change one voice.
Try this
Q1. Which type of motion has one voice staying on the same pitch while the other moves? [1 point]
- Cue. Oblique motion; it is safe because a held note cannot form parallel perfect intervals.
Q2. Why are parallel octaves forbidden between the soprano and bass? [2 points]
- Cue. Two voices an octave apart moving to another octave blend into a single reinforced line, destroying the independence of the parts.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)1 marksSection I (multiple choice, written). The soprano rises from C to D while the bass falls from E to D. What type of motion is this between the outer voices? (A) parallel (B) similar (C) contrary (D) obliqueShow worked answer →
The correct answer is (C) contrary.
Contrary motion is when the two voices move in opposite directions: here the soprano moves up (C to D) while the bass moves down (E to D). Contrary motion is the safest and most prized type because it almost never creates parallel perfect intervals.
(A) parallel motion keeps the same interval and same direction. (B) similar motion moves in the same direction but by different intervals. (D) oblique motion has one voice staying still while the other moves. The trap is calling opposite-direction motion "similar"; similar means same direction, contrary means opposite.
AP 2022 (style)2 marksSection II (free response, part-writing). A soprano line moves G to A and the bass moves C to D, both rising a major second. State the harmonic interval at each point and explain why this two-chord motion breaks a voice-leading rule.Show worked answer →
A 2-point error-spotting question.
(1 point) The first interval between bass C and soprano G is a perfect fifth; the second interval between bass D and soprano A is also a perfect fifth.
(1 point) Both voices rise by the same interval into a second perfect fifth, so this is parallel perfect fifths, which is forbidden. Two perfect fifths in a row in similar or parallel motion blur the independence of the voices.
Markers reward identifying both intervals as perfect fifths and naming the error as parallel perfect fifths. The fix is to change one voice so the second interval is no longer a perfect fifth, ideally using contrary or oblique motion.
Related dot points
- Topic 4.2 SATB Voice Leading: apply the rules of range, spacing, doubling, smooth motion and tendency-tone resolution when writing four-part harmony.
A focused answer to AP Music Theory Topic 4.2, covering the four-voice ranges, the spacing rule (no more than an octave between adjacent upper voices), doubling guidelines, the ban on parallels and voice crossing, and resolving the leading tone and tendency tones, with a worked voicing.
- Topic 4.3 Harmonic Progression, Functional Harmony, and Cadences: explain tonic, predominant and dominant function, the normal direction of progressions, and the four cadence types.
A focused answer to AP Music Theory Topic 4.3, covering functional harmony (tonic, predominant, dominant), the normal flow tonic to predominant to dominant to tonic, and the four cadences (perfect authentic, imperfect authentic, half, plagal, with the deceptive cadence), with a worked cadence analysis.
- Topic 3.5 Roman Numerals and SATB: label diatonic chords with Roman numerals showing root and quality, and arrange chord tones in the SATB four-voice texture.
A focused answer to AP Music Theory Topic 3.5, covering Roman numeral analysis (case shows quality, figures show inversion), the diatonic numerals of major and minor keys, the SATB four-voice layout and ranges, and how to spell a chord across four voices, with a worked analysis.
- Topic 4.4 Voice Leading with Seventh Chords: part-write the dominant seventh and other seventh chords in root position, resolving the chordal seventh and leading tone correctly.
A focused answer to AP Music Theory Topic 4.4, covering part-writing the dominant seventh in root position, resolving the chordal seventh down by step and the leading tone up, the option of an incomplete chord to avoid parallels, and preparing the seventh, with a worked resolution.
- Topic 1.3 Half Steps and Whole Steps: identify, construct and correctly spell half steps and whole steps, including diatonic and chromatic half steps.
A focused answer to AP Music Theory Topic 1.3, covering the half step as the smallest Western interval, whole steps, diatonic versus chromatic half steps, correct letter-name spelling, and the keyboard layout, with worked spelling.
Sources & how we know this
- AP Music Theory Course and Exam Description — College Board (2024)