The Mason–Dixon Line (or "Mason and Dixon's Line") is a demarcation line between four U.S. states, forming part of the borders of Pennsylvania, Maryland, Delaware, and West Virginia (then part of Virginia). It was surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon in the resolution of a border dispute between British colonies in Colonial America. Popular speech, especially since the Missouri compromise of 1820 (apparently the first official usage of the term "Mason's and Dixon's Line"), uses the Mason-Dixon line symbolically as a cultural boundary between the Northern United States and the Southern United States (Dixie).citation needed]
Maryland and Pennsylvania both claimed the land between the 39th and 40th parallels according to the charters granted to each colony. The 'Three Lower Counties' (Delaware) along Delaware Bay moved into the Penn sphere of settlement, and later became the Delaware Colony, a satellite of Pennsylvania.
In 1732 the proprietory governor of Maryland, Charles Calvert, 5th Baron Baltimore, signed an agreement with William Penn's sons which drew a line somewhere in between, and also renounced the Calvert claim to Delaware. But later Lord Baltimore claimed that the document he signed did not contain the terms he had agreed to, and refused to put the agreement into effect. Beginning in the mid-1730s, violence erupted between settlers claiming various loyalties to Maryland and Pennsylvania. The border conflict between Pennsylvania and Maryland would be known as Cresap's War.
The issue was unresolved until the Crown intervened in 1760, ordering Frederick Calvert, 6th Baron Baltimore to accept the 1732 agreement. As part of the settlement, the Penns and Calverts commissioned the English team of Charles Mason and Jeremiah Dixon to survey the newly established boundaries between the Province of Pennsylvania, the Province of Maryland, Delaware Colony and parts of Colony and Old Dominion of Virginia.
After Pennsylvania abolished slavery in 1781, the western part of this line and the Ohio River became a border between free and slave states, although Delaware remained a slave state.