In Western classical music, obbligato (Italian pronunciation: [obbliˈɡaːto], also spelled obligato) 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, but it is often used as an alternative spelling in the US. 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).