AP Music Theory: Four-Part Chorale Writing Rules

Mastering four-part chorale writing is a cornerstone of the AP Music Theory curriculum, serving as both a practical application of harmonic principles and a direct assessment of your analytical and compositional skills. This style, modeled after J.S. Bach’s chorales, demands strict adherence to voice-leading conventions that ensure clarity, independence of lines, and satisfying harmonic progression. Success in this area is non-negotiable for the free-response section of the exam, where you must harmonize melodies and bass lines with precision and musicality.

The Foundation: Voice Ranges and Spacing

Before you can write a single chord, you must understand the framework for the four vocal parts: soprano, alto, tenor, and bass (SATB). Each has a standard, acceptable range to ensure singability and distinct tonal quality. The soprano typically sings the highest melodic line, the alto fills the inner harmony below, the tenor provides the upper male voice, and the bass anchors the entire texture with the root of the chord. Exceeding these ranges makes a part unperformable and breaks the stylistic idiom.

Equally critical is managing vertical spacing. The golden rule is to keep the distance between the soprano and alto, and between the alto and tenor, within one octave. This creates a cohesive sound in the upper three voices. However, the distance between the tenor and bass can exceed an octave without issue. A common spacing error is voice crossing, where a lower voice moves above a higher voice (e.g., the alto leaps above the soprano), which muddies the distinct vocal lines you are trying to maintain.

The Cardinal Rules of Voice Leading

Smooth, independent motion between chords is the essence of good voice leading. The primary goal is for each of the four voices to move as little as possible—preferably by step—to the nearest available chord tone in the next harmony.

The most famous prohibitions concern parallel perfect intervals. You must avoid parallel fifths and parallel octaves (and their compound equivalents like parallel twelfths) between any two voices. These intervals, when moved in parallel motion, destroy the independence of the voices by making them sound fused together. For example, if the bass and tenor both move from C to G, creating parallel fifths (C–G to D–A), the effect is considered crude and is strictly forbidden in this style. Direct fifths and octaves (where the outer voices leap to a perfect interval in similar motion) are also generally avoided, especially at cadences.

Tendency tones are scale degrees that have a strong pull to resolve in a specific direction, and managing them is non-negotiable. The most critical is the leading tone (scale degree 7 in a major or harmonic minor key), which must resolve upward by step to the tonic. If the leading tone is in the soprano, this creates a strong melodic cadence; if it’s in an inner voice, the rule still applies. The chordal seventh (the seventh of any seventh chord, like V7 or ii7) is another tendency tone with an irresistible downward pull; it must resolve down by step. Forgetting to resolve a chordal seventh downward is one of the most common and costly errors.

Doubling Guidelines and Chord Construction

In a four-voice texture, you only have four notes to represent a chord that may have only three unique pitches (a triad). Therefore, one note must be doubled. The safest practice is to double the root of the triad, especially in root-position chords. Doubling the fifth is also generally acceptable. You should avoid doubling the leading tone or the chordal seventh, as this amplifies their tendency to resolve and can create problematic parallels. In first-inversion chords (chords in 6/3 position), doubling the soprano voice or the bass (which is the chord’s third) is often a good strategy to ensure smooth voice leading.

When writing seventh chords in four parts, you include the root, third, fifth, and seventh. This means no doubling is necessary, as you have four unique chord tones. However, you may occasionally need to omit the fifth of a seventh chord to facilitate smooth voice leading, in which case you would double the root. The resolution of the seventh remains paramount.

Practical Harmonization and AP Exam Strategy

On the AP exam, you will be asked to harmonize a given melody or bass line using four-part chorale style. Your process should be methodical. First, analyze the given line for implied harmonies and cadence points. A strong authentic cadence (V–I or V7–I) at the phrase’s end is almost always required. Use common progressions like ii–V–I or I–vi–ii–V–I as your backbone.

When writing, work from the bass line up. If the bass is given, determine the chord for each note, then write the soprano as a strong, singable counter-melody to the bass. If the melody is given, determine the implied harmony for each note and construct a bass line that moves primarily by root movements of fourths/fifths or by step. Then fill in the inner voices (alto and tenor), whose primary job is to complete the harmony while moving as smoothly as possible. Always check your work backwards: scan for parallel fifths/octaves by comparing each pair of voices, and verify the resolution of every leading tone and chordal seventh.

Common Pitfalls

1. Unresolved Tendency Tones: Letting a leading tone stagnate or moving a chordal seventh upward. Correction: The leading tone (Ti) must go to Do. The chordal seventh must resolve down by step.
2. Spacing and Range Errors: Placing the alto and tenor more than an octave apart from the voices above them, or writing a part outside its singable range. Correction: Keep the distance between Soprano-Alto and Alto-Tenor within an octave. Memorize the standard ranges (e.g., Tenor: C3–G4) and stay within them.
3. Parallel and Direct Perfect Intervals: Creating consecutive fifths or octaves between any two voices, or having the outer voices leap to a perfect interval in similar motion. Correction: After writing, do a dedicated check comparing each pair of voices (S-A, S-T, S-B, A-T, A-B, T-B) for parallel motion. If you find parallels, re-voice one of the chords, often by changing the doubling or using a different chord inversion.
4. Poor Doubling: Doubling an active tone like the leading tone, or omitting the third of a chord (which defines its quality as major or minor). Correction: In root-position triads, double the root. In first-inversion triads, consider doubling the soprano or bass. Never omit the third.

Summary

• Voice Leading is Paramount: Strive for smooth, predominantly stepwise motion in all four voices, avoiding the forbidden parallel fifths and octaves.
• Tendency Tones Are Law: The leading tone must resolve up to the tonic, and the chordal seventh must resolve down by step—without exception.
• Structure Supports Sound: Keep voices within their standard ranges and maintain close spacing (within an octave) between the upper three voices.
• Double for Stability: In triads, prioritize doubling the root, avoid doubling tendency tones, and never omit the chord’s third.
• Practice with Purpose: Mastery comes from repeatedly harmonizing melodies and bass lines, checking your work systematically for the errors outlined above, and internalizing common harmonic progressions.