A duet is a musical composition for two performers in which the performers have equal importance to the piece. It is often used to describe a composition involving two vocalists. It differs from a harmony, as the performers take turns playing a solo section rather than playing simultaneosuly. In classical music, the term is most often used for a composition for two singers or pianists. A piece performed by two pianists performing together on the same piano is referred to as "piano duet" or "piano four hands". A piece for two pianists performing together on separate pianos is referred to as a "piano duo".
"Duet" is also used as a verb for the act of performing a musical duet, or colloquially as a noun to refer to the performers of a duet. The word is also occasionally used in reference to non-musical activities performed together by two people.
When Mozart was young, he and his sister Marianne played a duet of his composition at a London concert in 1765. The four-hand, described as a duet, was in many of his compositions which included five sonatas; a set of variations, two performers and one instrument, and a sonata for two pianos. The first published sonata or duet was in 1777.