Delaware was first defined in a March 22, 1682 gift from King Charles II to his brother James, duke of York. It included land within 12 miles of New Castle, “extending south to Cape Lopen.” ...

On the Maryland-Delaware border along Route 13, Delmar is a town split by the two states. It has roughly 1,780 Delaware residents and 3,210 on the Maryland side, according to 2017 census data.

However, the two states share two land borders. In Pennsville, near Fort Mott, a section of land that is mainly marsh area, approximately 1.7 miles in length and 0.6 miles wide is owned by Delaware.

Jeopardy Champ Ken Jennings explains why Delaware has the 12-mile circle—a very unusual border, consisting of a perfect circle carved out of southern Pennsylvania.