Delaware is the second-smallest state in the United States, Rhode Island being the smallest. Delaware is bounded to the north by Pennsylvania, to the east by the Delaware River, New Jersey and the Atlantic Ocean and to the west and south by Maryland. Small portions of Delaware are also situated on the far, or eastern, side of the Delaware River Estuary, and these small parcels share land boundaries with New Jersey. The state of Delaware, together with the Eastern Shore counties of Maryland and two counties of Virginia, form the Delmarva Peninsula, a geographical unit stretching far down the Mid-Atlantic Coast. 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, DelawareNew 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 (border)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. :Main articles: Twelve-Mile Circle, The Wedge (border)The Wedge, Mason-Dixon line, Transpeninsular Line Source: http://en.wikipedia.org/wiki/Delaware