the wedge-shaped piece at the summit of an arch, regarded as holding the other pieces in place.
something on which associated things depend: the keystone of one's philosophy.
Also called keystone sack. Baseball Slang. second base ( def 1 ).

1630–40; key1 + stone

2. basis, principle, foundation, linchpin. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
keystone (ˈkiːˌstəʊn)
1.  headstone, Also called: 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 10th Edition
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Word Origin & History

"stone in the middle of an arch, which holds up the others," 1637, from key (1) in fig. sense of "that which holds together other parts." Fig. sense is from 1641. 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 crew of officers in the slapstick films 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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