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An arched srtucture or a semi-circular end of a wall. In byzantine architecture it means the semicircular, usually barrel-vaulted, niche at the east end of a basilica. The side aisles of a basilica may also end in an apse, but it is always in the central apse where the altar is placed. It was separated from the main church by a barrier, the templon, or the iconostasis. Its ground plan on the external side could be semicircular, rectangular or polygonal.

peristyle
A colonnade surrounding a building or a courtyard .

praepositos
(from lat. praepositus sacri cubiculi) The praepositos or praipositos (πραιπόσιτος του ιερού κουβουκλίου or του ευσεβεστάτου κοιτώνος) was a grand chamberlain by the Early Byzantine period the highest official, usually Eunuch, in the imperial service. After the 6th C. his functions were assumed by the parakoimomenos. The dignity involved in palace ceremony and disappeared until the end of 11th C.