The performers at a concert are usually raised above the level of the audience on a stage. Concerts may be held in concert halls which are built for the purpose, or they may be held in any other suitable large building such as a school hall, a nightclub, a barn or a large house or castle. Some concerts are given to very large audiences in the open air. They may take place in a field or in a stadium. The music for these “open-airs” is usually amplified by loudspeakers so that large audiences can hear it.