Harmony and voice leading

Voice leading is the art of moving from one chord to the next so that each individual voice — soprano, alto, tenor, bass — moves smoothly. The chords are vertical objects, but the music is heard as four (or more) horizontal lines, and good voice leading is what makes those lines singable. Where harmony asks which chord comes next, voice leading asks how the singers (or the fingers) get from this chord to that one.

The rules of voice leading were codified in the eighteenth century for four-part chorale writing — Bach's chorales remain the canonical study material — and they remain the foundation of any keyboard accompaniment, any string-quartet texture, any choral arrangement. Even in styles that flout them deliberately (jazz, twentieth-century atonal music), the underlying logic still holds: each voice should be a coherent line that someone could sing.

The first principle of voice leading is to move each voice the smallest distance possible. If two adjacent chords share notes, those notes should be held in the same voice — this is called common-tone retention. Voices that must move should move by step (a second) wherever possible; leaps of a third or more should be the exception, not the rule.

The bass is the standard exception: the bass line drives harmonic progression and so is allowed to leap freely, especially by the fifths and fourths that mark cadences.

The most famous voice-leading rule is the prohibition on parallel fifths and parallel octaves. If two voices are a fifth apart and both move up a step, they remain a fifth apart — this is a parallel fifth, and it sounds bare and hollow because the two voices momentarily fuse into a single thicker line.

Parallel octaves are even worse: the two voices become one. The prohibition exists because the goal of four-voice writing is to keep four voices audibly distinct, and parallels reduce the count.

The fix is contrary motion (one voice up, one voice down) or oblique motion (one voice still, one voice moving). Composers from Bach to Brahms scrutinised every chord change for hidden parallels and rewrote them.

Modern popular music tolerates parallels freely — the power chord is a deliberate parallel fifth — but in any style aiming for the sound of independent voices, the prohibition still applies. Even in pop, the bass and the lead vocal almost never run in parallel for long.

Bad (parallel 5ths):              Better (contrary motion):
  Soprano:  C  →  D                Soprano:  C  →  D
  Alto:     F  →  G                Alto:     F  →  E
  (5th apart, both up)              (no parallels — voices diverge)

Some scale degrees have strong tendencies. The leading tone (the seventh degree of the major scale) wants to resolve up to the tonic. The fourth degree, especially when it is the seventh of a dominant chord, wants to resolve down to the third.

Sevenths in general resolve down by step; suspensions resolve down by step; the augmented fourth expands outward, the diminished fifth contracts inward. These tendencies are not arbitrary preferences — they are the whole grammar of tonal voice leading.

• Leading tone (7̂) — resolves up to tonic.
• Sevenths of seventh chords — resolve down by step.
• Suspensions — resolve down by step.
• Augmented fourth (tritone, °5 spelled as +4) — expands outward.
• Diminished fifth (tritone, +4 spelled as °5) — contracts inward.

Honouring these tendencies is what makes voice leading sound right. A leading tone that fails to resolve sounds unfinished. A seventh that leaps free sounds harsh. The whole grammar of tonal harmony is built on the idea that every dissonant note has somewhere it wants to go, and the composer's job is to let it go there. Beginners constantly want to add the most dramatic chord; experts spend more time worrying about how each voice resolves.

In a four-voice texture you have four notes to assign and only three notes in a triad — so one note is doubled. The convention: double the root of root-position triads; double the soprano of first-inversion triads (because doubling the bass makes the inversion sound like a root-position chord); never double the leading tone (because both voices would want to resolve up to the tonic, producing parallel octaves).

In four-voice writing the upper three voices (soprano, alto, tenor) are kept within an octave of each other; the bass is allowed to sit further below. Wider spacing between adjacent upper voices produces a hollow sound; closer spacing produces a thick sound.

Most chorale-style writing keeps the soprano and alto within a sixth, the alto and tenor within an octave, and the tenor and bass however far apart they need to be. Watching the spacing across a piece is half the visual character of a Bach chorale.

Every rule has exceptions. Hidden fifths between the outer voices are forbidden in chorale writing but tolerated when the soprano moves by step. Crossed voices (alto above soprano, say) are usually avoided but appear when they produce smoother lines than the alternative.

The dominant-seventh chord allows its fifth to be omitted to permit a stronger resolution. None of these are licence for sloppy writing; they are surgical exceptions used by composers who have already internalised the standard rules.

Twentieth-century music abandoned many of these conventions. Debussy used parallel fifths and ninths as colour. Stravinsky used unresolved dissonance as structure. Schoenberg rejected the idea of voice leading entirely in favour of motivic logic.

But even in those styles, melodic shape and stepwise motion remained important — the rules changed, but the underlying need for each line to be musical did not. Studying classical voice leading first gives you the vocabulary to recognise what later composers chose to abandon and what they chose to keep.

The classic exercise is to harmonise a Bach chorale melody in four voices, following the rules above. Pick a Bach chorale you do not know; cover the lower three voices; harmonise the soprano line yourself; compare with Bach's solution. After fifty chorales the rules become reflexes, and you start hearing voice-leading errors in your own playing the moment you make them. There is no faster way to understand four-voice writing.