Unison
Prime, the same note in several voices.
What it means
Unison (German Einklang) designates the simultaneous sounding of the same note in several voices or instruments. Considered as an interval it is the prime: the distance zero, the ratio 1:1. It counts as a perfect consonance, indeed as its limiting case, because the two notes merge completely.
Musical use
In polyphonic writing the unison is therefore delicate. Two voices that hold it for a longer time or run in parallel in it cease to be two voices — parallel primes are forbidden for the same reason as parallel octaves. As a passing moment, by contrast, it is unobjectionable: voices meet on one note and part again, which occurs especially at cadences and endings.
As a means of scoring, the unison has great effect. A unison — several instruments or voices on the same line — sounds decided and unified; after a many-voiced texture it acts like a narrowing, like a common resolution after many opinions. In orchestral works, passages led in unison often open a movement or mark a break.
At the same time the unison is the most demanding task for intonation: deviations that hardly stand out in thirds are audible here at once as beats. On the piano this is a constructional principle — several strings per note, whose slight detuning makes the sound lively and whose excessive deviation ruins it.
In strict counterpoint it counts, like the octave, as a permitted sonority of two voices only at the beginning and end, because the independence of the voices is lost. In orchestration it is used deliberately to reinforce a line through several instruments.
Other names
Einklang is the German designation; unisono (Italian) is the playing instruction to play a part together in unison or in octaves. As an interval the unison is called prime (German Prime).
Performance practice
Practise passages led in unison with particular care: same attack, same length, same articulation, and matched vibrato — even small differences split the unison into two lines. Tune to the lowest note and listen for the disappearance of the beats. And do not take unison passages louder than necessary; their effect lies in unity.