A clerestory is a part of a structure which rises above surrounding an adjoining roofs, allowing light inside the building through copious large windows. Clerestories can vary widely in size, from small features in homes to massive vaulted structures in Gothic cathedrals, but in all cases, they make a structure feel less dim and gloomy than it might otherwise. Typically, constructing a clerestory adds considerably to the price of a building, since it requires special structural support and extra glass.
The oldest examples of clerestories can be found in the temples of Ancient Egypt, where architects added a clerestory to the interior rooms of their temples in order to bring light inside. Without a clerestory, inner rooms would have been totally cut off from the outside, making them very dim and oppressive. The Ancient Greeks picked up the trick and passed it on to the Romans, and it diffused across Europe from there, reaching its height in Gothic construction.
Cathedrals and old churches often have very excellent examples of clerestories, typically over the nave, the large central portion of the church used for worship, and many Roman basilicas and other public structures such as baths also have clerestories. Clerestories can also be seen in homes, both ancient and modern, and they are extremely common in factories. Factory builders like to include clerestories because they flood the floor with natural light, saving energy. In homes, one advantage of a clerestory is that it admits light and fresh air without reducing the privacy of the residents of the house.
Because a clerestory demands extra time and construction material when it is installed, a structure with a clerestory will generally sell at a higher price than a comparable structure without one. This can be useful to remember when looking at and considering structures for sale; many people find that the clerestory is worth the extra cost, since it brings so much light and air into the structure. Especially in urban areas, this architectural feature can be a great benefit.
You can identify a clerestory by the fact that it is typically lined with windows, to maximize the light which reaches the inside. When viewed from the outside, a clerestory is also very visible because it is elevated above the rest of the roof. Depending on the style of design, the clerestory may be ornamented with carvings, latticework, and stained glass, or it may be left more simple.