Pennsylvania – The Keystone State

The word “keystone” comes from architecture and refers to the central, wedge-shaped stone in an arch, which holds all the other stones in place. The application of the term “Keystone State” to Pennsylvania cannot be traced to any single source. It was commonly accepted soon after 1800.
At a Jefferson Republican victory rally in October 1802, Pennsylvania was toasted as “the keystone in the federal union,” and in the newspaper Aurora the following year the state was referred to as “the keystone in the democratic arch.” The modern persistence of this designation is justified in view of the key position of Pennsylvania in the economic, social, and political development of the United States.
Source: http://www.phmc.state.pa.us/