1/1
| Interval information |
(perfect) prime,
1st harmonic,
1st subharmonic,
fundamental
harmonic,
highly composite harmonic
(Shannon, [math]\displaystyle{ \sqrt{nd} }[/math])
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 cents (or any other logarithmic measure such as octaves, 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.
As an interval region
| ← | Interval region | Comma and diesis → |
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 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 (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.