An auditorium is a room built to enable an audience to hear and watch performances. For movie theatres, the number of auditoria (or auditoriums) is expressed as the number of screens. Auditoria can be found in entertainment venues, community halls, and theatres, and may be used for rehearsal, presentation, performing arts productions, or as a space. It is commonly found at large hotels and convention centres though many other establishments, including even hospitals, have one. Sometimes other rooms are modified for large conferences such as arenas or concert halls. Aircraft have been fitted out with conference rooms. Conference rooms can be windowless for security purposes. An example of one such room is in the Pentagon, known as the Tank.