Obbligato

In Western classical music, obbligato (Italian pronunciation: [obbliˈɡaːto], also spelled obligato^[1]) usually describes a musical line that is in some way indispensable in performance. Its opposite is the marking ad libitum. It can also be used, more specifically, to indicate that a passage of music was to be played exactly as written, or only by the specified instrument, without changes or omissions. The word is borrowed from Italian (an adjective meaning mandatory; from Latin obligatus p.p. of obligare, to oblige); the spelling obligato is not acceptable in British English,^[2] but it is often used as an alternative spelling in the US.^[3] The word can stand on its own, in English, as a noun, or appear as a modifier in a noun phrase (e.g. organ obbligato).