Generally refers to small domes, often resembling overturned cups or bulbs, placed on a circular, polygonal, or square base or on small pillars or a glassed-in lantern or skylight. A cupola is typically used to crown a turret, roof, or larger dome. The term may also refer specifically to the inner vault of a dome. When these structures are intended to be used as lookouts, prefer "belvederes"; for windowed superstructures on roofs or domes used to admit light or air to the space below, use "lanterns (roof appendages)." To refer specifically to onion-shaped domes of any size, use "onion domes."