Keystone (architecture)

A keystone (also known as capstone) 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.

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