High Quality Content by WIKIPEDIA articles! The so-called Twelve-Mile Circle is not actually a circle but the compound arcs of two or more different circles that have been imperceptibly feathered together to form most of the boundary between the Commonwealth of Pennsylvania and the State of Delaware in the United States. It is nominally a circle with a supposed ? yet in fact only approximate and variable ? twelve-mile radius, centered in the town of New Castle, Delaware. In 1750, the center of the circle was fixed at the cupola of the courthouse in New Castle. (However, subsequent changes have been based on a different center point also, several thousand feet to the northwest of the courthouse.) The Twelve-Mile Circle continues into the Delaware River. A small portion of the circle, known as the "Arc Line," also forms part of the Mason-Dixon line, separating Delaware and Maryland. And two other small portions, although not actually demarcated until 1934, form parts of the boundary between the states of Delaware and New Jersey.