Definition
Three-part counterpoint combines three independent lines (for example soprano, alto and bass, or two upper voices and a bass). It is the decisive step between two-part writing, where every interval is a choice, and four-part writing, where the harmony is complete: in three parts every chord is necessarily incomplete (a triad has three notes, but you often double one), so you constantly have to decide which note to omit. Fux treats it in the Gradus ad Parnassum (1725) with the same five species as two-part writing; Bach takes it to its peak in the Sinfonias (three-part Inventions) BWV 787–801 and in the organ trio sonatas BWV 525–530.
Harmonic rules
- On strong beats aim for the complete triad (root, third, fifth). If you must omit, omit the fifth, never the third.
- Preferred doubling: the root. Avoid doubling the leading tone and altered notes.
- Chords in first inversion (6/3) are very useful: they sound complete even with a doubling and create singable bass lines.
- The fourth between the upper voices is allowed (it does not involve the bass). The fourth with the bass remains a dissonance.
- Beginning and end: perfect chord or bare octave/fifth at the beginning, complete triad or doubled root with third at the end (in strict style often an ending without the third, or with a Picardy third in minor).
Melodic and motion rules
- No parallel fifths and octaves between any pair of voices (with three voices there are three pairs to check: S–A, S–B, A–B).
- Hidden octaves and fifths are forbidden between the outer voices when the soprano leaps; tolerated if they involve inner voices.
- Distances: the upper voices normally within an octave of each other; the bass may move away up to a twelfth or more.
- Voice crossing: rare and brief.
- Rhythmic independence: if one voice moves in eighth notes, the others hold longer notes or move in a complementary way.
The species in three parts
- First species: three voices note against note, one of which is the cantus firmus.
- Mixed species: the cantus firmus in long notes, one voice in second species (two notes), one in third (four notes), or one in fourth (suspensions). This is the most instructive exercise, because each voice has its own rhythm.
- Fifth species (florid) against two voices: prepares the writing of inventions and fugues.
Example in C major (cadence)
| Voice | m.1 | m.2 | m.3 | m.4 |
|---|---|---|---|---|
| Soprano | E | F | D | C |
| Alto | C | A | B | C |
| Bass | C | D | G | C |
Chords: I (C–E with doubled root) – ii (D–F–A complete) – V (G–B–D complete, fifth in the soprano) – I. Contrary motion between soprano and bass in bars 1–2, leading tone B rising to C.
Real examples
- Bach, Sinfonia No. 9 in F minor BWV 795: three subjects in triple invertible counterpoint, intense chromaticism.
- Bach, Organ Trio Sonatas BWV 525–530: two hands and pedal, each voice with an equal role.
- Corelli, trio sonatas op. 1–4: two violins plus basso continuo, the model of Baroque three-part texture.
- Modern arranging: the classic three-part horn section (trumpet, tenor sax, trombone) or three-part vocal harmonies in pop (lead + third above + fifth below) follow the same priorities: third always present, contrary motion between outer voice and bass.
Exercise for a producer
- Take a 4-chord progression (for example C Am F G).
- Write a stepwise bass or one moving in leaps of a fourth/fifth, then a high voice in contrary motion.
- Add the middle voice by choosing the note that completes the triad (often the missing third).
- Make the voices rhythmically independent: one in quarter notes, one in eighth notes, one in half notes. Assign the three channels to different timbres (for example a flute, a clarinet, a bass) and check in the Mixer that none masks the others.
Related topics
Four-part SATB writing, Four-part counterpoint, Invertible counterpoint at the octave, Fugue, Two-part invention, Voicing.