CLERESTORY, CLERESTOREY or CLEARSTORY, in architecture, any wall of a room carried higher than the sur rounding roofs so that windows can be pierced in it to light the room. In a large building, where interior walls are far from the outside of the building, some such method of lighting the central part becomes necessary, and the use of the clerestorey appears as early, at least, as the 18th dynasty in Egypt, under which the great hypostyle hall of the temple at Karnak was built. This had a central range of columns, higher than those on either side, to allow clerestoreys to be built of pierced stone slabs. In Roman architecture, many great halls were thus lighted, usually groined vaults over the central hall allowed of large semicircular windows being built above the side roofs; e.g., S. Maria degli Angeli, at Rome (the tepidarium of the Baths of Diocletian) and the basilica of Constantine, at Rome. Similarly, the walls under the side arches of S. Sophia at Constantinople, are clerestorey walls. It was, how ever, in the Romanesque and Gothic churches of the middle ages, that the clerestorey idea received its most adequate expression. See p. 8o1 for illustrations of Clerestories. (See BYZANTINE AND