Which U.S. state borders Maryland, Pennsylvania, and New Jersey?

This small Mid-Atlantic state is uniquely positioned, acting as a geographic linchpin for its three neighbors (Review). The majority of its land border is shared with Maryland to the south and west, as both states occupy the Delmarva Peninsula. To the north, it meets Pennsylvania along a famous and unusual boundary known as the Twelve-Mile Circle. The state's entire eastern edge is defined by the Delaware River and Delaware Bay, which separates it from New Jersey.

The history of these borders adds fascinating context. The Twelve-Mile Circle is a true arc surveyed in 1763, centered on the courthouse in New Castle, making it one of the only circular state boundaries in the country. This arc also created a small, disputed piece of land called "The Wedge," which was officially assigned to the state in 1921.

Furthermore, the water boundary with New Jersey is not a simple line down the middle of the river. Due to a colonial-era deed, the state of Delaware technically owns the entire surface of the Delaware River up to the low-water mark on the New Jersey side. This unique arrangement has been upheld by the U.S. Supreme Court and gives the state significant control over the crucial waterway.