A clerestory window is a big window or sequence of tiny windows that run along the top of a structure's wall, generally towards the roof line. Clerestory windows are a sort of "fenestration," or glass window layout, that can be seen in both residential and commercial buildings. A clerestory wall frequently rises over adjacent roofs. It is so named because the early Christian church had one or more large windows in the roof, where the sun would pass through during prayer meetings or other services held there. There are two main types of clerestory windows: flat plate and multiple-light transoms. Flat plate clerestories have rectangular or trapezoidal openings cut into the wall surface, usually with plain glass panels inside. These are the most common type of clerestory used for office space, schools, and hospitals. Multiple-light transoms have several smaller rectangular or round windows set within a larger central window, often with stained glass. They are commonly found in churches, but some non-religious structures such as gymnasiums also use them. The number of lights in a transom window varies depending on the design; sometimes as many as 12 or more individual windows might be used together in a single structure. Clerestory windows allow in more light than ordinary single-glazed windows while providing visual appeal and allowing some natural light into the building.