Clerestory

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The church of St. Nicolai, Stralsund. The clerestory is the level between the two green roofs, reinforced here by flying buttresses.

In architecture, a clerestory (/ˈklɪərstɔːri/, KLEER-staw-ree; lit. clear storey, also clearstory, clearstorey, or overstorey) is a high section of wall that contains windows above eye level. The purpose is to admit light, fresh air, or both.

Historically, clerestory denoted 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.

Similar structures have been used in transportation vehicles to provide additional lighting, ventilation, or headroom.