Pennsylvania's nickname is The Keystone State because it was the middle colony of the original 13 colonies and because Pennsylvania has held a key position in the economic, social, and political development of the United States. The state song of Pennsylvania also mentions the state nickname in the second verse: "Birthplace of a mighty nation, Keystone of the land." The word keystone is from architecture, it describes the central wedge-shaped stone in an arch which holds all the other stones in place:
Three of our most treasured documents were written in Pennsylvania: the US Constitution, the Declaration of independence, and the Gettysburg address.
Pennsylvania license plate featuring the "keystone" symbol - photo by Norm Russo (used by permission)