"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.
[kee-stohn] 
