Voussoir

A voussoir /vuˈswɑːr/ is a wedge-shaped element, typically a stone, used in building an arch or vault.

Although each unit in an arch or vault is a voussoir, two units are of distinct functional importance: the keystone and the springer. The keystone is the center stone or masonry unit at the apex of an arch. The springer is the lowest voussoir on each side, located where the curve of the arch springs from the vertical support or abutment of the wall or pier.