A duet is a musical composition for two performers in which the performers have equal importance to the piece, often a composition involving two singers or two pianists. It differs from a harmony, as the performers take turns performing a solo section rather than performing simultaneously. A piece performed by two pianists performing together on the same piano is a "piano duet" or "piano four hands". A piece for two pianists performing together on separate pianos is a "piano duo".
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.
Duets have always been a part of the structure of operas. Early 16th-century operas such as L'Orfeo and L'incoronazione di Poppea involve duets throughout the performance. In 17th-century Italy duets were often used in comic scenes within serious operas. In Baroque France the duet was popular in tragedies, such as songs of vengeance and confrontation. The love duet was characterized by singing in close harmonies of 3rds and 6ths, symbolizing unity after conflict.
Throughout the 20th century duets have been common in the popular music of the era. Some songs were written to be heard as conversations, such as "Baby, It's Cold Outside". Others were performed around a theme, for example New York in "Empire State of Mind". Occasionally duets are an improvisation between artists, such as "Under Pressure". David Bowie and Freddie Mercury reportedly composed the lyrics in a day by improvising together.