Twelve-Mile Circle
The Twelve-Mile Circle is an approximately circular arc which forms most of the boundary between Pennsylvania and Delaware. It is not actually a circle, but rather a combination of different circular arcs that have been feathered together. It is nominally a circle with a supposed—yet in fact only approximate and variable— 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. 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. Two other small portions, although not actually demarcated until 1934, form parts of the boundary between the states of Delaware and New Jersey. Although the Twelve-Mile Circle is often claimed to be the only territorial boundary in the United States that is a true arc, many cities in the South (such as Plains, Georgia) also hav ...
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