An apse is a vaulted or domed recess or projection on a structure, often at the east end of a church, that houses the altar. The word comes from a Latin term meaning "a shelter," and an apse can be found in almost all major Christian churches throughout the world. Cathedrals usually have more than one apse. They are often called "cathedral" because they contain a shrine to the Virgin Mary called the "cathedral" or "mother of God." The word "museum" comes from the same root word as "cathedral." In addition to the main west front, which faces the street, most cathedrals have two other fronts: one toward the nave (the body of the church without the chancel or sanctuary) and one toward the choir (the portion of the church where sacred music was sung). These additional fronts often include large numbers of windows and often serve as the main entrance into the building. The cathedral in Rouen, France, is a good example of this type of architecture. It has three apses with beautiful stained-glass windows and several other interesting features. Cathedrals were originally built with only one apse.