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Family
May these two voices move like this?
The general laws that govern any number of voices moving together: the four kinds of relative motion, the prohibitions on parallel perfect intervals, the handling of hidden fifths and octaves, spacing, doubling, and the melodic conduct of a single line. Every other family in this reference assumes these.
Two voices can relate in only four ways: parallel motion (same direction, interval kept), similar motion (same direction, interval changing), oblique motion (one holds, one moves), and contrary motion (opposite directions).
Two voices may not move to a perfect fifth from a perfect fifth in the same direction: consecutive fifths.
Two voices may not move to an octave from an octave in the same direction: consecutive octaves.
Similar motion into a perfect fifth or octave — hidden or direct fifths and octaves — is restricted even though the interval is only reached, not repeated: the parallel is implied by the gesture.
Each voice keeps its registral lane: crossing — an upper voice sounding below a lower one — is avoided in strict writing because the ear assigns lines by register and loses the thread when they trade places.
The upper three voices stay close: soprano to alto, and alto to tenor, each within an octave of the next, while the gap between tenor and bass may open wider — the bass is a foundation, not a neighbor.
In four voices something must be doubled: the root first, the fifth second, the third sparingly — and in an inverted chord, prefer doubling a tone other than the bass when the bass carries the third.
The leading tone is never doubled.
At a cadence the leading tone rises to the tonic — that half-step ascent is the resolution the dominant exists to deliver, and in an outer voice it is obligatory.
The chordal seventh falls by step.
The perfect fourth leads a double life: between two upper voices it is a consonance and moves freely, but against the bass it is a dissonance and must be treated — prepared and resolved like the suspension it usually is, or passed through in motion.
A single line does not move by an augmented second — or any augmented interval — in the strict style: the interval is hard to sing and harder to hear as melody, its two tones implying different harmonic worlds.
A leap borrows energy the line must repay: after any large leap the melody turns back by step in the opposite direction.
A line avoids outlining the tritone: when the tones at a phrase segment’s turning points — where it changes direction or comes to rest — stand a tritone apart, the ear hears the forbidden interval even though no single leap made it.
When a tone and its chromatic alteration sound in successive chords, they belong in the same voice: F in the alto followed by F♯ in the alto is a line inflecting; F in the alto followed by F♯ in the soprano is a false relation — two voices contradicting each other about one degree, and the ear flags the contradiction even across the octave.