Keystone (architecture)

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

Keystone from the palazzo Borgazzi (Milan, Italy)

Rib vault keystone with boss, in church of Virgin Mary in Chełmno

Dropped keystone at Colditz Castle

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.^[1] This makes a keystone very important structurally.^[2]^[3] 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.^[4] 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.

[edit] Figurative use

The term is used figuratively to refer to the central supporting element of a larger structure, such as a theory or an organization, without which the whole structure would collapse.^[3] Example: Trade is the keystone of modern civilization.

The U.S. state of Pennsylvania is called the "Keystone State" because of its central location and commercial and political importance among the 13 colonies. The notched keystone shape is sometimes used as a logo for the state. From this, the keystone shape became the logo of the Pennsylvania Railroad (PRR), the shoulder sleeve insignia of the 28th Infantry Division of the Pennsylvania National Guard, Little League Baseball (founded in Williamsport), and of Heinz Ketchup (headquartered in Pittsburgh).^{[citation needed]} The PRR's passenger and mixed traffic locomotives had keystone numberplates. The keystone shape is also used on Pennsylvania's state route markers.

[edit] Gallery

The York Minster Chapter House ceiling
A boss depicting Jesus Christ decorates the keystone in the rib-vaulting at Chapel of St. Anne in Malbork , 14th century.
Ornamental keystone over portal
Bossed keystone in the ceiling of an apse chapel (Toulouse Cathedral)
Symbolic shape
Arches with equal-size voussoirs and keystones, Mosque of Cordoba, Spain