Mason–Dixon line - Wikipedia

On October 18, 1767, the Mason Dixon line, the boundary between Maryland and Pennsylvania, was agreed upon. From the article:

"The Mason–Dixon line, also called the Mason and Dixon line or Mason's and Dixon's line, was surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon in the resolution of a border dispute involving Maryland, Pennsylvania, and Delaware in Colonial America. It is still a demarcation line among four U.S. states, forming part of the borders of Pennsylvania, Maryland, Delaware, and West Virginia (originally part of Virginia before 1863). Later it became known as the border between the North and South. Before the Missouri Compromise, the line (west of Delaware) marked the northern limit of slavery in the United States.

Maryland's charter of 1632 granted the Calverts land north of the entire length of the Potomac River up to the 40th parallel. A problem arose when Charles II granted a charter for Pennsylvania in 1681. The grant defined Pennsylvania's southern border as identical to Maryland's northern border, but described it differently, as Charles relied on an inaccurate map. The terms of the grant clearly indicate that Charles II and William Penn believed the 40th parallel would intersect the Twelve-Mile Circle around New Castle, Delaware, when in fact it falls north of the original boundaries of the City of Philadelphia, the site of which Penn had already selected for his colony's capital city. Negotiations ensued after the problem was discovered in 1681. A compromise proposed by Charles II in 1682, which might have resolved the issue, was undermined by Penn receiving the additional grant of the "Three Lower Counties" along Delaware Bay, which later became the Delaware Colony, a satellite of Pennsylvania. Maryland considered these lands part of its original grant.[1]

The conflict became more of an issue when settlement extended into the interior of the colonies. In 1732 the proprietary governor of Maryland, Charles Calvert, 5th Baron Baltimore, signed a provisional agreement with William Penn's sons, which drew a line somewhere in between and renounced the Calvert claim to Delaware. But later, Lord Baltimore claimed that the document he had 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 would be known as Cresap's War.

Progress was made after a Court of Chancery ruling affirming the 1732 agreement, but the issue remained unresolved until Frederick Calvert, 6th Baron Baltimore ceased contesting the claims on the Maryland side and accepted the earlier agreements. Maryland's border with Delaware was to be based on the Transpeninsular Line and the Twelve-Mile Circle around New Castle. The Pennsylvania–Maryland border was defined as the line of latitude 15 miles (24 km) south of the southernmost house in Philadelphia (on what is today South Street). 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, and Delaware Colony.[1]

In 1779, Pennsylvania and Virginia agreed "To extend Mason's and Dixon's line, due west, five degrees of longitude, to be computed from the river Delaware, for the southern boundary of Pennsylvania, and that a meridian, drawn from the western extremity thereof to the northern limit of the said state, be the western boundary of Pennsylvania for ever."[2]

After Pennsylvania abolished slavery in 1781, the western part of this line and the Ohio River became a border between slave and free states, with Delaware[3] retaining slavery until the Thirteenth Amendment was ratified in 1865."