Sometimes these windows are very small, being mere quatrefoils or spherical triangles. In large buildings, however, they are important objects, both for beauty and utility. The windows of the clerestories of Norman work, even in large churches, are of less importance than in the later styles. In Early English they became larger; and in the Decorated they are more important still, being lengthened as the triforium diminishes. In Perpendicular work the latter often disappears altogether, and in many later churches, as at Taunton, and many churches in Norfolk and Suffolk, the clerestories are close ranges of windows.
The term is equally applicable to the Egyptian temples, where the lighting of the hall of columns was obtained over the stone roofs of the adjoining aisles, through slits pierced in vertical slabs of stone. The Romans also in their baths and palaces employed the same method, and probably derived it from the Greeks; in the palaces at Crete, however, light-wells would seem to have been employed.