The Mason–Dixon Line (or "Mason and Dixon's Line") is a line of demarcation between four states in the United States. It forms part of the borders of Pennsylvania, West Virginia, Delaware, and Maryland, surveyed when they were still British colonies. After Pennsylvania began abolishing slavery within the Commonwealth, in 1781, the western part of this line, and the Ohio River, became most of the border between the free and slave states (Delaware, however, east of the Mason-Dixon Line remained a slave state). Popular speech, especially since the Missouri compromise of 1820, uses the Mason-Dixon line symbolically as a supposed cultural boundary between the Northern United States and the Southern United States.The men surveying this property were Charles Mason, and Jeremiah Dixon.
The Mason–Dixon line was surveyed between 1763 and 1767 in the resolution of a border dispute in colonial North America. Maryland and Pennsylvania both claimed the land between the 39th and 40th parallels according to the charters granted to each colony. In the meantime, '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 not finally resolved 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.