keystone

The keystone (shown in red) of an arch Dropped keystone at Colditz Castle

A keystone (or capstone) is the wedge-shaped stone at the apex of a masonry arch or typically round-shaped one at the apex of a vault. In both cases it is the final piece placed during construction and locks all the stones into position, allowing the arch or vault to bear weight. In arches and vaults (such as quasi-domes) keystones are often enlarged beyond the structural requirements and decorated. A variant in domes and crowning vaults is a lantern. A portion of the arch surrounding the keystone is called a crown.