Editing Keystone (architecture) (section)

{{Short description|Wedge-shaped stone at the apex of a masonry arch}}{{About|the architectural element||Keystone (disambiguation)}}
{{Redirect|Key stone}}
[[File:L-Karniesbogen (keystone).png|thumb|The keystone (shown in red) of an arch]]
[[File:Entrance of Colditz Castle chapel.jpg|thumb|Dropped keystone at [[Colditz Castle]]]]

A  '''keystone''' (or '''capstone''') is the wedge-shaped stone at the [[apex (geometry)|apex]] of a masonry [[arch]] or typically round-shaped one at the apex of a [[Vault (architecture)|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.<ref name=ching>{{cite book | last = Ching | first = Francis D.K. | title = A Visual Dictionary of Architecture | publisher = John Wiley & Sons, Inc. | year = 1995 | location = New York | pages = 12 | isbn = 0-471-28451-3 }}</ref><ref>{{cite web |url= http://www.pitt.edu/~medart/menuglossary/keystone.htm |title= Glossary of Medieval Art and Architecture – Keystone |publisher= [[University of Pittsburgh]] |access-date= 2007-06-25 }}</ref><ref name="merriam">{{cite web |url= http://www.merriam-webster.com/dictionary/keystone |title= keystone |publisher=Merriam Webster |access-date= 2007-06-25 }}</ref> 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 (architecture)|lantern]]. A portion of the arch surrounding the keystone is called a [[Crown (arch)|crown]].{{sfn | Wilkins | 1879 | p=292}}

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

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 [[voussoir]]s, due to its position at the apex.{{citation needed|date=April 2022}} Old keystones can decay due to [[weathering]] and vibration, a condition known as [[bald arch]].