Opera is a form of theater in which the dialog is entirely, or mostly, sung instead of spoken and which makes extensive use of an orchestra both for accompanying the singers and for solely instrumental sections. Along with the music and acting, opera also typically makes extensive use of costumes and scenery, as in conventional theater, and, additionally, sometimes contains dance or ballet. This most complex and costly of the performing arts has been a major part of Western classical music since the late sixteenth century.