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keystone

[kee-stohn] /ˈkiˌstoʊn/
noun
1.
the wedge-shaped piece at the summit of an arch, regarded as holding the other pieces in place.
2.
something on which associated things depend:
the keystone of one's philosophy.
3.
Also called keystone sack. Baseball Slang. second base (def 1).
Origin
1630-1640
1630-40; key1 + stone
Synonyms
2. basis, principle, foundation, linchpin.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for key stone

keystone

/ˈkiːˌstəʊn/
noun
1.
Also called headstone, quoin. the central stone at the top of an arch or the top stone of a dome or vault
2.
something that is necessary to connect or support a number of other related things
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for key stone

keystone

n.

"stone in the middle of an arch, which holds up the others," 1630s, from key (n.1) in figurative sense of "that which holds together other parts" + stone. Figurative sense is from 1640s. Pennsylvania was called the Keystone State because of its position (geographical and political) in the original American confederation, between northern states and southern ones. Keystone cops were the bumbling police in the slapstick silent movies produced by Keystone Company, formed by Canadian-born U.S. film director Mack Sennett (1884-1960) in 1912.

Online Etymology Dictionary, © 2010 Douglas Harper
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10
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