A vault that rises from two circular-plan walls (as above the walls of an ambulatory) is called an annular vault. An example is the dome of a church. The word "vault" comes from the Latin vallum, meaning "wall." So, a vault is a walled-in space below ground or within a building structure. Vaults are used to provide protection from the elements and for aesthetic purposes. Vents in roofs allow water to escape while keeping out heat and cold air. Windows let in light and provide views outside the building while allowing fresh air into living spaces. Doors lead people from one room or section of a building to another. There are several different roof types used in construction. The three most common are flat, sloped, and hipped. On a flat roof, there is no rise or fall from edge to edge. It is level all around its perimeter. This type of roof is used on single-story buildings with no interior rooms or areas to obstruct traffic or rainwater runoff. In a sloped roof, one side is higher than the other.