keystone (n.)
"stone in the middle of an arch (typically the uppermost stone), which holds up the others," 1630s, earlier simply key (1520s), from key (n.
What does keystone mean?
1 : the wedge-shaped piece at the crown of an arch that locks the other pieces in place — see arch illustration. 2 : something on which associated things depend for support determination, a keystone of the puritan ethic— L. S. Lewis.
Who invented the keystone?
It is true that Egyptian, Babylonian, Greek and Assyrian civilizations used arches for underground structures such as drains and vaults. However, it was the Roman civilization (1000 B.C.E. - 500 C.E.) that first began using a keystone (also called a capstone) in their arches.
