Unison is not the absence of an interval but a special case. When several voices sing in unison, the sound gains particular weight — a “closed-ranks” quality that does not exist in multi-voice textures.
Unison is the foundation of choral singing in liturgy (znamenny chant, Gregorian chant), military marches, and mass songs. In the symphony orchestra, tutti in unisono — the moment when every instrument plays a single melody — is one of the most powerful expressive devices: a kind of “all of us, together.”
Strictly speaking, the unison is the “zero” interval between two pitches of the same height. When instruments or voices play in unison, small divergences of intonation and timbre do not vanish but merge into a fuller, “choral” sound — an effect used by both symphony orchestras and rock bands when they double a part. The octave unison (the same note in different octaves) sounds especially powerful and solemn.
