Why is Pennsylvania called the Keystone state?

Though the nickname's origin is unknown, it's certain that it was in use around, or shortly after, 1800. It's reported that Pennsylvania was toasted as "...the keystone of the federal union" at a Republican presidential victory rally for Thomas Jefferson in 1802. Regardless of its origin, the nickname has come to represent Pennsylvania's geographic, economic, social and political impact on development of the United States.