Keystone (architecture)

This article is about the architectural element. For other uses, see Keystone (disambiguation).

Voussoir stones 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.

Keystones, as a hallmark of strength or good architecture, or their suggested form are sometimes placed in the centre of the flat top of doors, recesses and windows for decorative effect, so as to form an upward projection of a lintel.

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 commonly mark the intersections of any two or more arched ribs.

For aesthetics, keystones are often larger than ribs in vaults and many of the voussoirs (arch stones) in arches, 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. Numerous examples are found in the work of Sebastiano Serlio, a 16th-century Italian Mannerist architect.

The U.S. state of Pennsylvania calls itself the "Keystone State", because during early American history, it held a crucial central position among the Thirteen Colonies geographically, economically, and politically, like the keystone in an arch. A keystone is a symbol of Pennsylvania, being used in the logos of many Pennsylvania government departments and on Pennsylvania car license plates.

Gallery

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  Keystone from the palazzo Borgazzi (Milan, Italy)
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  Keystone much enlarged for decorative effect, and carrying a coat of arms, Barcelona
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  The York Minster Chapter House rib-vault ceiling with central and peripheral keystones
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  A boss depicting Jesus Christ decorates the keystone in the rib-vaulting at Chapel of St. Anne in Malbork, 14th century.
• 
  Bossed keystone in the ceiling of an apse chapel (Toulouse Cathedral)
• 
  Arches with equal-size voussoirs and keystones, Mosque of Cordoba, Spain
• 
  Dropped keystone on a German church
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  Doors and windows with decorated keystones, Munich
• 
  A range of plain but enlarged keystones by Giulio Romano for his house in Mantua

See also

• Architectural sculpture
• Coping (architecture)
• List of classical architecture terms
• Oculus compression ring

External links

Media related to keystones at Wikimedia Commons