The Clerestory As Direct Light Into The Nave

A clerestory is a row of windows set high in the nave wall, above the roofline of the lower side aisles that flank it, positioned specifically so that daylight can reach the nave directly rather than filtering in only through the lower aisle windows and the arcade separating aisle from nave. In basilical church design, where the nave rises considerably higher than the aisles on either side, the section of wall exposed above the aisle roof and below the nave's own roof creates exactly the band of surface a clerestory occupies, so the element exists only where this difference in height between nave and aisle is present, making the clerestory a direct architectural consequence of the basilica's stepped cross-section rather than an independent addition to it.

Because clerestory windows sit well above eye level and above the columns and arcade of the nave arms below, the light they admit tends to fall from a high angle down into the center of the nave, a quality of illumination noticeably different from the softer, more diffuse light typically admitted through lower aisle or west-facade windows, and this high, direct light has long been noted by architectural historians as contributing significantly to the particular atmosphere of large basilical church interiors.