Keystone (architecture)
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In architecture, a keystone is the stone at the top of a masonry arch. It is popularly but falsely believed that the keystone is more important structurally than any of the other wedge-shaped stones (voussoirs) that make up the arch. In fact, removal of any of the voussoirs would cause the arch to collapse. The keystone is often larger than the other voussoirs, but this is purely for aesthetic purposes.
[edit] Figurative use
A notched keystone was also the logo of the Pennsylvania Railroad, which operated throughout the north-eastern and Midwestern United States, Pennsylvania being known as the "keystone state."
In the Bible the parable which referred to the "stone that the builders rejected" is the "head cornerstone," or "keystone." There is no direct translation of keystone from the idiomatic terms in ancient Aramaic, Hebrew, and Greek: presumably, there were no "keys" in those times, or the term had not been yet applied as widely as it is today.
