The Twelve-Mile Circle

Most of the border between Delaware and Pennsylvania is not a straight line but a curve, part of a circle with a radius of 12 miles centered on the courthouse cupola in New Castle, Delaware. The circle dates to 1682, when the Duke of York granted William Penn the lands within 12 miles of New Castle, and surveyors first laid it out in 1701. It gives Delaware's northern edge the rounded shape that makes the state instantly recognizable on a map.

The circle also reaches across the Delaware River. Within its arc, Delaware owns the river right up to the low-water line on the New Jersey shore, including some land built up on the Jersey side from dredged sand. The arrangement has led to repeated court fights. In New Jersey v. Delaware (2008), the U.S. Supreme Court upheld Delaware's authority over the river inside the circle, which let Delaware block a proposed natural gas terminal on the New Jersey bank.

• New Jersey
  The Garden State, densely settled between two cities borders
• Pennsylvania
  The Keystone State, ridge-and-valley heart of Appalachia borders
• Delaware
  The First State, a low coastal sliver on the Atlantic located in
• The Delaware Wedge
  A sliver of land claimed by three states until 1921 related