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In music, a unison is an interval, the ratio of 1:1 or 0 halfsteps and zero cents. Two tones in unison are considered to be the same pitch, but are still perceivable as coming from separate sources. The unison is considered the most consonant interval while the near unison is considered the most dissonant. The unison is also the easiest interval to tune. Composer Kenneth Gaburo wrote for three voices singing unison in his piece The Flow of (u).

The unison is abbreviated as P1.

A pair of tones in unison can have different "colors" (timbres), i.e. come from different musical instruments or human voices. Voices with different colors have, as sound waves, different waveforms. These waveforms have the same fundamental frequency but differ only in the amplitudes of their higher harmonics.

When several people sing together, as in a chorus, the simplest way for them to sing is to sing in "one voice", in unison. If there is an instrument accompanying them, then the instrument must play the same notes being sung by the singers (in order for there to be unison). Otherwise the instrument is considered a separate "voice" and there is no unison. If there is no instrument, then the singing is said to be a cappella. Music in which all the notes sung are in unison is called monophonic.

From this sense can be derived another, figurative, sense: if several people do something "in unison" it means they do it simultaneously, in tandem, in lockstep. Related terms are "univocal" and "unanimous".

Monophony could also conceivably include more than one voice which do not sing in unison but whose pitches move in parallel, always maintaining the same interval of an octave. A pair of notes sung one or a multiple of an octave apart are almost in unison, due to octave equivalency.

When there are several voices which do not usually sing in unison, then the result is polyphony. The simplest polyphony is homophony, where the voices sing notes at different pitches but with the exact same rhythm. An example is a barbershop quartet.

See also:

In politics, UNISON is the British trade union of the public service.
In the 1970s Unison was a short lived civil defence outfit supported by right wing politicians such as Ross McWhirter.
Unison is also the title of a music album by singer Celine Dion.