Unison

A unison is the "interval" between two notes of exactly the same pitch: two singers on the same G, or a violin and a flute playing the same A. Its full name is the perfect unison, written P1, and it measures zero half steps. Some older books call it the prime.

Unison on the page

When two voices share one staff and meet on the same note with the same length, the note is usually written once with two stems, one up and one down. When the two notes have different lengths, they are written side by side.

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When each player reads a separate part, both parts simply show the same note. In orchestral string parts, unis. (unisono) tells players who had been split into two lines (divisi) to play one line together again.

Singing and playing in unison

"In unison" also describes a whole passage in which everyone plays or sings the same melody. Strictly, that means the same pitches. In everyday use it includes doubling at the octave: when a mixed choir sings a melody "in unison", the men usually sing an octave below the women. Plainchant (Gregorian chant) is sung in unison, and many bands play a unison riff, guitar and bass on the same line, for extra weight.

Why a unison is hard to play in tune

Two notes in unison have a frequency ratio of 1:1. If they are even slightly apart, you hear beating: a slow pulse or wobble in the sound. As the two pitches move together, the beats slow down and finally disappear. String players, guitarists and choirs use this to tune, and it is why a unison passage exposes every intonation problem.

Augmented unison

Raise or lower one of the two notes by a half step while keeping its letter, as in G–G♯, and the interval becomes an augmented unison: one half step, the same distance as a minor second but spelled with a single letter name.

The Intervals chart starts with the unison, and Hear the Pulse Between Two Tones lets you hear two almost-unison tones beating.