keystone (architecture)

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.

Architecture

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.

A "dropped keystone" is one where the keystone projects lower than the other voussoirs. Following Giulio Romano, 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.

Metaphor

Keystone is often used metaphorically for an essential part on which the whole depends.

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, in Pennsylvania state route signs, and on Pennsylvania car license plates, and the Shoulder Sleeve Insignia of the Pennsylvania National Guard's 28th Infantry Division (United States), a Red Keystone, first adopted in WWI in October 1918.

Gallery

Image:8500 - Milano - Palazzo Borgazzi (1829) - Dettaglio - Foto Giovanni Dall'Orto, 31-Aug-2007.jpg, Keystone from the palazzo Borgazzi (Milan, Italy)
File:Barcelona. Generalitat Palace. Door to Carrer del Bisbe. C. 1638. Pere Pau Ferrer, architect. (18966422650) (cropped).jpg, Keystone much enlarged for decorative effect, and carrying a coat of arms, Barcelona
Image:Chapter House ceiling (crop 1).jpg, The York Minster Chapter House rib-vault ceiling with central and peripheral keystones
File:Middle keystone in the Chapel of St. Anne in Malbork showing Jesus Christ.jpg, A boss depicting Jesus Christ decorates the keystone in the rib-vaulting at Chapel of St. Anne in Malbork, 14th century.
Image:Toulouse Cathedral - Keystone 1.jpg, Bossed keystone in the ceiling of an apse chapel (Toulouse Cathedral)
Image:Spain Andalusia Cordoba BW 2015-10-27 13-54-14.jpg, Arches with equal-size voussoirs and keystones, Mosque of Cordoba, Spain
File:Bonn-Oberkassel Alte Evangelische Kirche Schlussstein.jpg, Dropped keystone on a German church
File:München-Altstadt Rumfordstraße 23 490.jpg, Doors and windows with decorated keystones, Munich
File:Tor der Casa di Giulio Romano.JPG, A range of plain but enlarged keystones by Giulio Romano for his house in Mantua
File:Mark Master Keystone.gif, The keystone is the symbol of the Order of Mark Master Masons
File:Navicular bone12.png, As a metaphor, the navicular bone, shown in green, is known as the keystone of the foot

See also

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

Notes

External links

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Arches and vaults
Ornaments (architecture)
Architectural elements