The Mason–Dixon line, also called the Mason and Dixon line or Mason's and Dixon's line, is a demarcation line separating four U.S. states, constituting parts of the borders of Pennsylvania, Maryland, Delaware, and West Virginia. It was surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon as part of the resolution of a border dispute involving Maryland, Pennsylvania, and Delaware in the colonial United States. The dispute had its origins almost a century earlier in the somewhat confusing proprietary grants by King Charles I to Lord Baltimore (Maryland), and by his son King Charles II to William Penn.

Maryland is a state in the Mid-Atlantic region of the United States. The state borders Virginia to its south, West Virginia to its west, Pennsylvania to its north, Delaware to its east, the Atlantic Ocean, and the national capital of Washington, D.C. With a total area of 12,407 square miles (32,130 km2), Maryland is the ninth-smallest state by land area, and its population of 6,177,224 ranks it the 18th-most populous state and the fifth-most densely populated. Maryland's capital is Annapolis, and the most populous city is Baltimore. Occasional nicknames include Old Line State, the Free State, and the Chesapeake Bay State. It is named after Henrietta Maria, the French-born queen of England, Scotland, and Ireland during the 17th century.