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. 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.
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. 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). The PRR's passenger and mixed traffic locomotives had keystone numberplates. The keystone shape is also used on Pennsylvania's state route markers.
See also 
- Architectural sculpture
- Boss (architecture)
- Coping (architecture)
- List of classical architecture terms
- Oculus compression ring
- Pennsylvania Railroad
- Ching, Francis D.K. (1995). A Visual Dictionary of Architecture. New York: John Wiley & Sons, Inc. p. 12. ISBN 0-471-28451-3.
- "Glossary of Medieval Art and Architecture - Keystone". University of Pittsburgh. Retrieved 2007-06-25.
- "keystone". Merriam Webster. Retrieved 2007-06-25.
- "Windows and More About Arches". Retrieved 2007-06-25.
Media related to keystones at Wikimedia Commons
|Wikisource has the text of the 1911 Encyclopædia Britannica article Keystone.|