keystone

Keystone from the palazzo Borgazzi (Milan, Italy)

Vault keystone in church of Virgin Mary in Chełmno

A keystone is the architectural piece at the crown of a vault or arch which marks its apex, locking the other pieces into position.^[1] This makes a keystone very important structurally.^[2][3] In an arch, the keystone is usually larger than the voussoirs that make up the arch and may serve primarily an aesthetic purpose. Some say that a keystone is not as important structurally as the voussoirs, since the removal of any of the voussoirs would cause the arch to collapse but this is not necessarily true of the keystone.^[4]

Old keystones, crucial in medieval buildings, can decay due to vibration, a condition known as bald arch.

In a vault, keystones may mark the intersections of tracery.

Mannerist architects of the 16th century enjoyed building an arch with a slightly dropped keystone as seen below in example at Colditz Castle.

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: Language is the fractured keystone of arts and libertarian ideals.

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), 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.

Gallery

Multiple decorative keystones, Sorunda Church, Nynäsham	The York Minster Chapter House ceiling	Keystones with tracery at Lierne vault of the Liebfrauenkirche, Mühlacker 1482.	Dropped keystone at Colditz Castle in a Mannerist arch.
Ornamental keystone over portal	Vault keystone of an apse chapel (Toulouse Cathedral)	Symbolic shape

See also

Wikisource has the text of the 1911 Encyclopædia Britannica article Keystone.

• List of classical architecture terms
• Pennsylvania Railroad
• Architectural sculpture

Notes

1. ^ Ching, Francis D.K. (1995). A Visual Dictionary of Architecture. New York: John Wiley & Sons, Inc.. pp. 12. ISBN 0-471-82451-3. 
2. ^ "Glossary of Medieval Art and Architecture - Keystone". University of Pittsburg. http://www.pitt.edu/~medart/menuglossary/keystone.htm. Retrieved 2007-06-25. 
3. ^ ^{a b} "keystone". Merriam Webster. http://www.webster.com/cgi-bin/dictionary?sourceid=Mozilla-search&va=keystone. Retrieved 2007-06-25. 
4. ^ "Windows and More About Arches". http://www.brantacan.co.uk/keystones.htm. Retrieved 2007-06-25. 

External links

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