DUETTO. - The term (diminutive of duo) does not properly designate a musical genre, but various types of compositions consisting of two voices or parts.
In modern common usage it means a piece for two voices with instrumental accompaniment, more rarely a composition in its own right, more frequently a lyrical or dramatic moment in theatrical opera.
A particular form of the d. is the religious one, as it appears, without instrumental accompaniment, in the Bicinia of the 16th century, whereas in Viadana’s Concerti ecclesiastici it appears with basso continuo accompaniment and in either concertante or imitative style. In the secular sphere the chamber d. acquired considerable importance in the 17th and 18th centuries, especially in Italy, when it was cultivated by F. Rasi, P. Quagliati, A. Stradella, G. B. Bononcini, A. Steffani, and G. M. Clari; in modern times Mendelssohn’s duets should be remembered. A final meaning of d. coincides with that of an instrumental composition for two instruments of the same type (e.g., for two violins), concertante in character when with orchestra, or soloistic in character when akin to the sonata form.