Delaware was one of the 13 colonies participating in the American Revolution and on December 7, 1787, became the first state to ratify the Constitution of the United States, thereby becoming known as The First State. The definition of the northern boundary of the state is unusual. Most of the boundary between Delaware and Pennsylvania was originally defined by an arc extending 12 miles from the cupola of the courthouse in the city of New Castle. This boundary, often referred to as the Twelve-Mile Circle, is the only nominally circular state boundary in the United States.