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Malmesbury Abbey, Wiltshire, England. The clerestory carries the clear glass windows at the top of the picture. The next level down (flood-lit, with rounded arches) is the triforium, the lowest level is the nave arcade.

Clerestory (IPA: /ˈklɪə(r)stɔəri/; lit. clear storey, also clearstory, clearstorey, or overstorey) is an architectural term denoting an upper level of a Roman basilica or of the nave of a Romanesque or Gothic church, the walls of which rise above the rooflines of the lower aisles and are pierced with windows. The Romans also used clerestories in their basilica-like baths and palaces, and probably derived the clerestory from the Hellenistic architecture of the Greeks. The clerestory originated in the temples of Egypt. It is also used to denote a style of railway rolling stock (predominantly passenger), for example the Great Western Railway Clerestory carriage of the Victorian era where the windows in the roof 'cupola' provided access to, and ventilation for, the vehicle's gas lighting.

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.

Stralsund, St Nicolai. The clerestory is the level between the two green roofs.

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.

Byzantine-style clerestory of the Monreale cathedral is elaborately covered with glass mosaic work.

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.

In the Minoan palaces of Crete, by contrast, light-wells seem to have been employed instead of clerestories.

The iconic road sign for a factory shows multiple clerestory windows

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 two distinct roof sections are joined by a clerestory window.

• Gothic architecture
• Cathedral architecture of the Western World