Voussoir

An element in an arch. Though each wedge shaped unit in an arch or vault is known as a voussoir, there are two specific voussoir components of an arch: the keystone and the springer. The keystone is the center stone or masonry unit at the apex of an arch. Often decorated, embellished or exaggerated in size, no true arching action occurs until this unit is in place. The springer is the lower most voussoir, located where the vertical support ends and the curve of the arch begins.