Keystone (n): A central stone at the summit of an arch, locking the whole together. The central principle or part of a policy, system, etc., on which all else depends.
Pennsylvania’s motto is: The Keystone State. This is because of its fundamental importance to the colonies as an entity in the in the 1600s. It was a center for commerce, energy production and became the site of the fledgling United States’ capitol and seat of government for a short period of time. Its symbol is the Keystone as it’s represented in the construction of bridge or structure supporting an arched doorway.
