The circular arch is the most common form of vaulting in Europe and America. It can be used to create rooms as large as churches, with a height up to 100 feet (30 m). The key feature of this type of vault is that it has no flat surface inside the room - instead, there are a number of steps leading up to a central point where the vault meets the ceiling. The term "Voussoir" comes from the French word "vouzoir", which means "to strike against". This refers to the role voussoirs play in supporting the arches they connect to prevent them from spreading too far and hitting the roof. Inside a church with a circular vault, the diameter of the circle is equal to two times the height of the room. For example, if the room is 20 feet high (6 m), then the vault's diameter will be 40 feet (12 m). There are several types of circular arches, depending on which part of the vault they support: pendentives, domes, or concaves.