Four-part writing, and the rules that keep the voices audible

Four-part harmony is the laboratory in which the rules of voice leading were discovered and where they are still learned: four lines — soprano, alto, tenor and bass — each singable, each heard as a line, all of them together forming the chords. The rules look like many and have a single reason: keeping the four voices independent to the ear.

Four voices, three notes

A triad has three notes and the choir has four voices: one note is doubled. The doubling rule in root position is to double the root; in first inversion, preferably double the soprano or the bass, avoiding the leading tone; in second inversion, double the bass, the fifth.

C major in four voices: bass C, tenor G, alto E, soprano C. The root appears twice, and the third, the most coloured note, only once; doubling the third thickens what should be thin, and the rule avoids that.

Spacing

Between soprano and alto, and between alto and tenor, an octave at most; between tenor and bass, whatever is needed. The reason is acoustic: the upper voices blend when they are close, and the bass can stand apart because low notes need space not to muddy.

The first chord has the three upper voices bunched in the low register and sounds opaque; the second spreads them and each is heard.

Movement between chords

The rules of movement are those of voice leading, applied strictly:

• Common tones stay put. The voice holding the common note does not move.
• The others move by step, to the nearest note.
• No parallel fifths or octaves. Two voices a fifth or an octave apart do not move in the same direction keeping that interval: the ear fuses them into one, and the texture loses a voice.
• No crossing. The alto does not go above the soprano or below the tenor.
• The leading tone rises to the tonic, and the dominant's seventh falls.

G major to C major, in four voices: the bass G leaps to the C; the tenor D falls to the C; the alto G stays; the soprano B rises to the C. No parallel fifths, no parallel octaves, the leading tone resolved, and the arrival chord with a doubled root and no fifth, as tradition accepts after a dominant.

Why the prohibitions exist

Parallel fifths and octaves are forbidden for a physical reason, not a matter of taste: two voices moving in parallel an octave apart sound like one thicker voice, and a fifth apart like one voice with a strange timbre. In both cases the ear stops counting four lines. The rule protects the independence of the voices, which is what four-part texture promises.

The procedure

Faced with a figured bass or a sequence of degrees, the routine is always the same. Write the bass. Choose the notes of the first chord with the right doubling and spacing. For each following chord, hold the common tones, move the others as little as possible, resolve the leading tone and the seventh, and check the parallels voice by voice. With practice, the checking becomes reflex and the rules disappear from consciousness, which is what they want.

CITonicFIVSubdominantG7V7DominantCITonic

What to take from here

Four voices, three notes, one doubled, and doubling the root is the default choice; the upper voices close together, the bass free; between chords, common tones held, the rest by step, no parallels, no crossing, the leading tone upward. All of it serves one thing: that the ear keeps counting four lines. It is voice leading with an exact brief, and the exercise that taught harmony to three centuries of musicians.