Unison (interval)
Unison, the first interval.
What it means
The unison (prime) is the interval between a note and itself — a distance of zero, the ratio 1:1. That it is called the “first” interval although it designates no distance is explained by the counting method: what is counted are the scale degrees involved, including the starting note, and with the prime that is only one. When two voices sound at the distance of a prime, one speaks of unison.
Musical use
As a perfect interval, the prime belongs among the perfect consonances and shares their properties: complete merging and the prohibition of parallel leading. Two voices that stay in unison are no longer two voices. As a passing moment, in which voices meet and separate again, the prime is, by contrast, unproblematic and common at cadences.
A special case is the augmented prime: the chromatic semitone, in which a note is altered without changing degree — C to C♯. It sounds like the minor second but means something different, for the minor second connects two different degrees. In the notation the augmented prime stays on the same line or in the same space, which makes it immediately recognizable.
As a means of scoring, unison is of great effect: several instruments on the same line sound decisive and unified. At the same time it is the most demanding task for intonation.
The prime is the interval between two notes of the same pitch. The perfect prime is distinguished from the chromatic semitone, which is designated as an augmented prime, for instance F–F♯. In interval theory, the prime serves as the zero point from which the degrees are counted.
Performance practice
Practise passages led in unison especially carefully — attack, length, and vibrato must match, otherwise the unison falls apart. Tune against the lowest note and listen for the beats disappearing. And with chromatic alterations, watch the notation: if the head stays at the same position, it is an alteration of the same note, not a step.