dome. Cupola, essentially a species of vault, constructed on a circular, elliptical, or polygonal plan, bulbous, segmental, semicircular, or pointed in vertical section. It can be built on top of a structure the plan of which is identical to that of the dome: if that wall is circular or elliptical it is a drum (often pierced with windows) as in a rotunda. However, domes usually provide a covering for a square-or rectangular-planned building or compartment, so there have to be adjustments to make the transition from the square to the circular, elliptical, or polygonal base of the cupola or dome. This can be achieved by means of pendentives (fragments (b) of a sail-vault (a, c) resembling a species of concave, distorted, almost triangular spandrels, rising up from the corner at the top of the right-angled compartment to the circular or elliptical base of the drum or cupola) or (d ) squinches (small arch or series of parallel arches of increasing radius spanning the angle of the square compartment). Both the drum and cupola will have a diameter the same dimension as the side of the square on which the whole structure stands.

Types of drum include:calotte: low cupola or saucer-dome of segmental vertical section;, like a skull-cap;cloister-vault: as domical vault below;domical vault: cloister-vault, not a true dome, but formed of four or more (depending on the shape of the base) cells or webs forming groins where they touch vertically and rising to a point;melon: as parachute below;Pantheon: low dome on the exterior, often stepped, resembling that of the Pantheon in Rome, and coffered on the interior, widely copied by Neo-Classical architects;parachute; melon, pumpkin, or umbrella dome standing on a scalloped circular base and formed of individual webs, segmental on plan, joining in groins or ribs. Each web has a concave interior and convex exterior so it resembles a parachute, rather than an umbrella;pumpkin: as parachute above;sail-dome (a): dome resembling a billowing sail over a square compartment with its diameter the same dimension as the diagonal instead of the side of the square below, enabling the structure to rise as though on pendentives but continuing without interruption. Pendentives are really parts of a sail-dome and themselves are a species of sail-vault;umbrella: as parachute above.