Pennsylvania's nickname is "the keystone state".

Note from history: The state was the middle colony of the original 13 colonies and played an important role in American history as it held a key position in the economic, social, and political development of the country. The state song of Pennsylvania also mentions the word "keystone" in its second verse: "Birthplace of a mighty nation, Keystone of the land."

This might be interesting: Originally, the word "keystone" was an architectural term. It describes the central wedge-shaped stone in an arch which holds all the other stones in place. Symbolic, isn't it?

More Info: www.statesymbolsusa.org