The Twelve-Mile Circle is an arc that marks most of the boundary between Delaware and which other US state?

The Twelve-Mile Circle is an approximately circular arc which forms most of the boundary between the Commonwealth of Pennsylvania and the State of Delaware in the United States. 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) 12-mile (19 km) 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.

Its existence dates back to 1681, when Charles II granted a deed to William Penn north of the already chartered Maryland. Charles created an exception, consisting of 12 miles around the town of New Castle and extending down the peninsula, as these lands were held by the Duke of York, who had won them in conquest from the Dutch colonists. Later, on August 24, 1682, the Duke granted these lands to Penn as well.