A dome, or cupola, is a spherical roof

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A dome, or cupola, is a spherical roof; it is ideal for a central structure (q. v.). Corbelled domes have existed since the Stone Age. They are constructed by laying the stones horizontally, each row overlapping the other, until the ceiling is closed by a key-stone. Some of the monumental round buildings of early Antiquity were vaulted in this manner (Treasury of Atreus, Mycenae). The style is still used to-day in some regions (Apulia). Properly vaulted domes probably originated in the Near East and were perfected by the Romans (The Pantheon, Rome, c. A. D. 120). Later, the dome became an important fcatu of Byzantine architectu (Hagia Sophia in Istanb C. A.D. 530). It was through Byzantium that the dome came to affect the architecture both of Islam and of Italy (Ravenna; St Mark's, Venice). Occasionally it even appears in France (Aquitaine, Périgueux). Elsewhere, the dome has only assumed importance again since the Renaissance ( Brunelleschi's dome in Florence, Michielangelo's dome for St Peter's, Rome).

Pendentive domes are of two types: in the one case, dome and pendentives are of the same hemisphere. The pendentive is simply the curved overhanging surface supporting the dome, each dome thus having four pendentives (cushion dome). In the second type of pendentive dome, the saucer dome, dome and pendentive consist of different hemispheres. This is an advanced form of the dome, already known in Byzantium, and rediscovered by Brunelleschi. Illustration 54 shows how the transition from a square section to an octagonal 'drum' -- to take the dome -- is effected. Often, the dome is surmounted by a small turret-like structure, the lantern (as in the dome of St Paul's, London). The horizontal segments between the pendentives are called lunettes.