1911 Encyclopædia Britannica/Duet
DUET (an adaptation of the Ital. duetto, from 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 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."