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 ribbed vaulting of Gothic architecture concentrated the weight and thrust of the roof, freeing more wall-space for larger clerestory fenestration. In Gothic churches, the clerestory is generally divided into bays by the vaulting shafts that continue the same tall columns that form the arcade separating the aisles from the nave.
Under the clerestory and above the arcade could be inserted an additional story, the triforium that helped dramatically increase the height of a Gothic nave. The triforium consists of a narrow passageway inserted in the wall beneath the windows of the clerestory and above the large gallery over the side aisles. The triforium is open to the nave through its own arcade, often doubling or tripling the number of arches to a bay.
In English churches, 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 Gothic 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.
At Hagia Sophia, for instance, the main dome rests on a drum pierced by clerestory lights.
The term "clerestory" is equally applicable to 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. Clerestory appeared in Egypt at least as early as the Amarna period.
By extension, "clerestory lights" are any rows of windows above eye level that allow light into a space. In modern architecture, clerestories provide light without distractions of a view or compromising privacy. Factory buildings are often built with clerestory windows (as illustrated on the sign); modern housing designs sometimes include them as well. Another example is the new Crosby Theatre of the Santa Fe Opera where the front and rear portions of the roof are joined by a clerestory window.