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A keystone is the wedge-shaped stone piece at the apex of a masonry arch, or the generally round 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 both arches and vaults, keystones are often enlarged beyond the structural requirements, and often decorated in some way. Keystones are often placed in the centre of the flat top of openings such as doors and windows, essentially for decorative effect.
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.
In a rib-vaulted ceiling, keystones may mark the intersections of two or more arched ribs. For aesthetic purposes, the keystone is sometimes larger than the other voussoirs, or embellished with a boss. Mannerist architects of the 16th century often designed arches with enlarged and slightly dropped keystones, as in the "church house" entrance portal at Colditz Castle (see image). Numerous examples are found in the work of Sebastiano Serlio, a 16th-century Italian Mannerist architect.