DUET (an adaptation of the Ital. duetto, from Lat. duo, two), a term in music for a composition for two performers, both either vocal or instrumental. The term is not properly applied to a composition for one voice and one instrument, the latter being regarded as an accompaniment, though in the modern evolution of this latter form of composition it often has the same character. Both parts must be of equal importance; if one is subordinated to the other it becomes an accompaniment and the work ceases to be a duet. Instrumental duets are written either for two different instruments, such as Mozart's duets for violin and piano, or for two similar instruments. Duets written for the pianoforte are either for two performers on two separate instruments or for two performers on the same instrument, when they are termed "duets à quatre mains."
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)