source, but not from Santa Sophia, which was not begun till 532 A.D. The central space is almost circular. Between each of the piers which support the octagonal clerestory at the base of the cupola is an apsidal recess, with three arches on the ground floor opening into the encircling aisle, and three upper arches opening into the gallery above the aisle. On the east side of the central space this arrangement is broken, and one tall arch opens into the chancel, which ends in a projecting apse, semi-circular inside, but a half octagon outside. The aisle with the gallery above thus occupies seven sides of the outer octagon, the eighth side being occupied by the western part of the chancel. [Illustration: Fig. 2. Plan of San Vitale, Ravenna: (1) _narthex_ with flanking turrets, as originally arranged; (2) central nave; (3) chancel and altar.] Sec. 8. Of the two types of plan, which can be studied so satisfactorily at Ravenna, the ordinary basilican type is the more convenient. The long nave provides the necessary accommodation for worshippers, the raised apse gives a theatre for the performance of service within view of everybody, the aisles facilitate the going and coming of the congregation, and prevent over-crowding. The centralised plan provides, it is true, a large central area conveniently near the altar; but the provision of a chancel or altar-space necessitates the grafting on the plan of a feature borrowed from the ordinary basilica, which, as at San Vitale, breaks the symmetry of the design. At Santa Sophia, the basilican chancel forms an indissoluble part of a centralised plan; but this feat is beyond the reach of an ordinary architect. Even at San Vitale the planning is highly complicated, and must be due to an architect of some genius. In addition to complications of design, the centralised plan raised questions of roofing which did not trouble the builders of the long wooden-roofed basilicas. The vaulted half-dome of the basilican apse was a simple matter, compared with the mighty dome of Santa Sophia and its cluster of abutting half-domes. It was in the centralised churches, with their domed vaults and the groined vaults of their aisles, that the history of medieval vaulting began. But, even when medieval masons had learned to regard the vaulting of their churches as the controlling principle of their art, they left the centralised plan almost entirely alone, and applied what it had taught them to the work of roofing basilicas with vaults of stone. We shall trace the influence of the centralised church as we proceed; but the influence of the basilica will be found to predominate in the history of medieval planning.