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The definition of the northern boundary of the state is highly unusual. Most of the boundary between Delaware and Pennsylvania is defined by an arc extending 12 miles (19 km) from the cupola of the courthouse in New Castle, and is referred to as the Twelve-Mile Circle. This is the only true-arc political boundary in the United States. This border extends all of the way to the low-tide mark on the New Jersey shore, which continues down the shoreline until it again reaches the twelve-mile arc in the south; then the boundary continues in a more conventional way in the middle of the main channel (thalweg) of the Delaware River Estuary. A portion of this arc extends into Maryland to the west, and the remaining western border is a tangent to this arc that runs a bit to the east. The Wedge of land between the arc and the Maryland border was claimed by both Delaware and Pennsylvania until 1921, when Delaware's claim was confirmed.

New Castle County is the northernmost of the three counties of the U.S. state of Delaware. As of 2000 its population was 500,265. The county seat is Wilmington. The center of population of Delaware is located in New Castle County, in the town of Townsend. It is the most affluent county in the state of Delaware.