View source for 1/1

{{Infobox Interval
| Name = (perfect) unison, (perfect) prime, 1st harmonic, 1st subharmonic, fundamental
| Color name = w1, wa unison
}}
The '''unison''' (interval ratio '''1/1''') is the [[interval]] between two tones that are identical in pitch. In the [[harmonic series]], 1/1 is the 1st [[harmonic]], and likewise in the [[subharmonic series]] 1/1 is the first [[subharmonic]] – this is because it acts as the fundamental to both series.

Measured in [[cent]]s (or any other logarithmic measure such as [[2/1|octave]]s, [[edo|edosteps]], etc.), the unison's size is exactly 0. This is because the distance between two identical pitches is zero. As such, the unison can be considered as a degenerate interval.

In [[just intonation]], 1/1 represents the base frequency from which an interval is measured.

In any [[temperament]], any [[comma]] [[tempering out|tempered out]] by that temperament is equated with 1/1, and thus intervals that differ by that comma in JI become equal in that temperament. For example, [[meantone]] sends [[81/80]] to the unison, and thus [[81/64]] and [[5/4]], which differ by [[81/80]] in JI, are equated to the same interval.

== As an interval region ==
{{Todo|complete section}}
{{Infobox interval region
| Name = Unison
| Cents lower = 0
| Cents upper = 0
| Cents upper wide = 6
| JI intervals = 1/1
| Complement = [[Octave]]
| Lower region =
| Higher region = [[Comma&nbsp;and&nbsp;diesis]]
}}
{{Wikipedia|Unison}}

As an interval region, the unison usually refers precisely to the 0-cent interval. However, there can be a tiny difference between any two intervals that are practically "the same note" (more pedantically, an extremely small [[Unnoticeable comma|comma]]), that might be considered a "unison" (or at least too small to be a meaningful interval). This range usually goes up to 3.5 cents, as that is the just-noticeable difference. 

In some practices, this bound goes up to about 6 cents, which is the most precisely one is expected to intonate a pitch on certain instruments, and is a bit smaller than a [[Kleisma (interval region)|kleisma]] (hence the kleisma's significance in the context of intonation). 

As a diatonic interval category, unisons represent [[subchromatic]] motions – i.e. the difference between a note and itself (though perhaps in a different tuning or using a non-diatonic accidental, though that's more generally covered by [[comma and diesis]]). Every note in every scale has a unison, which is that note itself.

In functional harmony, the unison over the root serves as the [[tonic]].

== See also ==
* [[Fundamental]]
* [[Octave reduction]]

[[Category:Unison| ]]<!-- main article -->
[[Category:1-odd-limit]]