A
keystone is the wedge-shaped stone piece at the
apex of a masonry
vault or
arch, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight. Although a masonry arch or vault cannot be self-supporting until the keystone is placed, the keystone experiences the least stress of any of the
voussoirs, due to its position at the apex. Old keystones can decay due to vibration, a condition known as
bald arch.