Keystone (architecture) - Figurative Use

Figurative Use

The term is used figuratively to refer to the central supporting element of a larger structure, such as a theory or an organization, without which the whole structure would collapse. Example: Trade is the keystone of modern civilization.

The U.S. state of Pennsylvania is called the "Keystone State" because of its central location and commercial and political importance among the 13 colonies. The notched keystone shape is sometimes used as a logo for the state. From this, the keystone shape became the logo of the Pennsylvania Railroad (PRR), the shoulder sleeve insignia of the 28th Infantry Division of the Pennsylvania National Guard, Little League Baseball (founded in Williamsport), and of Heinz Ketchup (headquartered in Pittsburgh). The PRR's passenger and mixed traffic locomotives had keystone numberplates. The keystone shape is also used on Pennsylvania's state route markers.