Four-part harmony is the SATB (soprano, alto, tenor, bass) texture used in AP Music Theory part-writing, where four voices spell each chord with correct accidentals, doubling, and spacing while following 18th-century voice-leading conventions (Topic 4.2).
Four-part harmony is music written for four distinct lines, labeled soprano, alto, tenor, and bass by their pitch position relative to each other. Per the CED (DES-1.C.1), figured bass and chorale harmonization exercises are notated in this SATB four-voice texture, which is why it's the default format for AP part-writing.
Here's the catch that makes it interesting. A triad only has three notes, but you have four voices to fill. That extra voice is where doubling rules come in (PIT-4.B.2). Double the root of a triad whenever voice leading allows, and use the third or fifth only when it produces better voice leading. On top of doubling, you follow spacing conventions (keep adjacent upper voices within an octave of each other) and voice-leading rules like varying the motion between outer voices and never writing more than three consecutive thirds or sixths between voices (PIT-4.C.1). Four-part harmony isn't just "four people singing." It's a rule system for stacking chords vertically while keeping each horizontal line smooth.
Four-part harmony lives in Unit 4, Topic 4.2 (SATB Voice Leading) and supports learning objectives 4.2.A through 4.2.D. You need to identify SATB lines in heard and notated music (4.2.A), spell and double chords correctly (4.2.B), apply voicing and spacing conventions (4.2.C), and extend all of it to first-inversion chords (4.2.D). This is the framework every part-writing skill in Units 4-7 builds on. Seventh chords, predominant harmony, secondary dominants, and modulation all get tested inside this same four-voice texture, so if your SATB fundamentals are shaky, every later unit inherits the problem.
Keep studying AP Music Theory Unit 4
Visual cheatsheet
view galleryVoice Leading (Unit 4)
Four-part harmony is the texture; voice leading is the rulebook for moving through it. Every SATB exercise is really a voice-leading exercise where you connect chords so each of the four lines moves smoothly and legally.
Counterpoint (Unit 4)
Counterpoint is two-voice, line-against-line writing, and four-part harmony is what you get when you scale that thinking up to four voices. The soprano-bass pair in SATB still behaves like a two-voice counterpoint framework, which is why the CED tells you to vary the motion between outer voices.
Parallel Fifths and Parallel Octaves (Unit 4)
With four voices moving at once, there are six voice pairs to check for forbidden parallels. Most points lost on SATB part-writing come from a parallel fifth or octave hiding between inner voices, so error detection means scanning every pair, not just soprano and bass.
Tendency Tones (Unit 4)
Doubling rules exist because of tendency tones. You never double the leading tone in four-part harmony because it has to resolve up to tonic, and doubling it forces either a parallel octave or an unresolved tendency tone.
Four-part harmony is tested two ways. Multiple-choice questions hit you with error detection and convention checks, like which vertical spacing violates 18th-century norms, which doubling is preferred in a root-position major triad, or what to double in a vii°6 or I6 chord. Free-response part-writing asks you to produce four-part harmony yourself, realizing a figured bass or Roman-numeral progression in SATB texture. The 2025 exam's harmonization question (SAQ Q7) asked you to complete a bass line under a given melody following 18th-century voice-leading procedures and label it with Roman and Arabic numerals. Scorers check chord spelling, doubling, spacing, and parallels in every voice pair, so the skill being graded is applying the conventions, not just knowing them.
Counterpoint is two independent melodic lines written against each other, judged note-against-note. Four-part harmony is a chordal texture where four voices sound together to spell complete harmonies. They share the same voice-leading DNA (no parallel fifths, resolve tendency tones), but counterpoint thinks horizontally first while four-part harmony balances horizontal lines against vertical chord spelling. On the AP exam, soprano-bass counterpoint skills feed directly into SATB part-writing.
Four-part harmony means SATB texture, with soprano, alto, tenor, and bass named by their pitch position relative to the other lines (DES-1.C.1).
Since a triad has three notes and you have four voices, one chord member gets doubled, and the default is to double the root whenever voice leading allows.
Never double the leading tone, because it must resolve up to tonic and doubling it creates parallels or an unresolved tendency tone.
Spacing matters: adjacent upper voices (soprano-alto, alto-tenor) should stay within an octave of each other, while the gap between tenor and bass can be wider.
Motion between the outer voices should vary, and no voice pair should move in more than three consecutive thirds or three consecutive sixths (PIT-4.C.1).
First-inversion chords follow the same normative 18th-century procedures, but the preferred doubling shifts (for example, double the third, the bass note, in vii°6).
It's the SATB (soprano, alto, tenor, bass) texture used for chorale harmonization and figured bass exercises, where four voices spell each chord following 18th-century doubling, spacing, and voice-leading conventions. It's the core of Topic 4.2 in Unit 4.
No. The CED says musical lines in instrumental or vocal pieces can be described as SATB based on pitch position, so a string quartet passage can be analyzed in four-part harmony just like a choir piece. The labels describe relative position, not who performs the line.
Double the root of a triad whenever voice leading allows; thirds and fifths can be doubled when they produce better voice leading. Special cases come up in first inversion, like doubling the third (the bass note) in vii°6, and you never double the leading tone.
Counterpoint is two melodic lines written against each other and judged note-against-note, while four-part harmony is a four-voice chordal texture judged on chord spelling, doubling, spacing, and voice leading all at once. The soprano-bass pair in SATB writing still works like counterpoint underneath.
Yes, heavily. Multiple-choice questions test spacing, doubling, and error detection in SATB texture, and the part-writing free-response questions require you to realize figured bass or Roman numerals in four voices. The 2025 exam's harmonization question asked for a bass line and Roman numerals following 18th-century voice-leading procedures.
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