A ballroom is a large room inside a building, the designated purpose of which is holding formal dances called balls. Traditionally, most balls were held in private residences; many mansions contain one or more ballrooms. In other large houses, a large room such as the main drawing room, long gallery, or hall may double as a ballroom, but a good ballroom should have the right type of floor.
Ballrooms are generally quite large, and may have ceilings higher than other rooms in the same building. The large amount of space for dancing, as well as the highly formal tone of events have given rise to ballroom dancing. The largest balls are now nearly always held in public buildings, and many hotels have a ballroom. They are also designed large to help the sound of orchestras carry throughout the whole room.
A special case is the annual Vienna Opera Ball, where, just for one night, the auditorium of the Vienna State Opera is turned into a large ballroom. On the eve of the event, the rows of seats are removed from the stalls, and a new floor, level with the stage, is built.