An adjective or noun referring to an essential instrumental part. The term is often used for a part ranking in importance just below the principal melody and not to be omitted. Obbligato is the opposite of Ad libitum when the latter qualifies the mention of a part in a title. On the title-page of Corelli's Concerti Grossi op.6, for example, the concertino parts are designated ‘obligato’ while the ripieno parts are described as ‘ad arbitrio, che si potranno radoppiare’ (as you wish, when you are able to double the parts). Used in connection with a keyboard part in the 18th century, obbligato designated a fully written-out part instead of a figured bass. Sometimes obbligato means simply independent, as in C.P.E. Bach's Orchester Sinfonien mit zwölf obligaten Stimmen (1780).
In music for voice with instruments, ‘obbligato’ refers to a prominent instrumental part in an aria or other number. The archetype of the obbligato part is the instrumental solo which, with a basso continuo, constitutes the accompaniment of vast numbers of late Baroque arias. The direct antecedents of the late Baroque phenomenon are to be found in the ...