A keystone is a central wedge in an arch that locks all other pieces of an arch in place. It is the part of an arch that all other parts depend upon.
Pennsylvania’s popular nickname, "The Keystone State," refers to this necessary element. Like most nicknames, it is not known, for certain, where this name originated, but there are a few interesting thoughts about how this nickname came to be. They are all based on the theme of the necessity of a keystone in a supporting structure.