The precise definition of "pendentive" has also been a source of academic contention, such as whether or not corbelling is permitted under the definition and whether or not the lower portions of a sail vault should be considered pendentives. Domes with pendentives can be divided into two kinds: "simple" and "compound". In the case of the "simple dome", the pendentives are part of the same sphere as the dome itself; however, such domes are rare. In the case of the more common "compound dome", the pendentives are part of the surface of a larger sphere below that of the dome itself and form a circular base for either the dome or a drum section.