The Keystone State

In answer to the question "Why is Pennsylvania called the Keystone State?", the Old Farmer's Almanac explains:

"A keystone is a wedge–shaped piece at the crown of an arch that locks the other pieces in place; it is a stone on which the associated stones depend for support. Geographically, Pennsylvania's central location along the arch of the 13 original states calls to mind a keystone. Politically, Pennsylvania played a vital role in holding together the states of the newly formed Union."

The attached map (nicked from Wikipedia) illustrates the central location that the Almanac refers to. Click here for the full-sized version.