Mason-Dixon Line, boundary between Pennsylvania and Maryland (running between lat. 39°43′26.3″N and lat. 39°43′17.6″N), surveyed by the English team of Charles Mason, a mathematician and astronomer, and Jeremiah Dixon, a mathematician and land surveyor, between 1763 and 1767. The ambiguous description of the boundaries in the Maryland and Pennsylvania charters led to a protracted disagreement between the proprietors of the two colonies, the Penns of Pennsylvania and the Calverts of Maryland. The dispute was submitted to the English court of chancery in 1735. A compromise between two families in 1760 resulted in the appointment of Mason and Dixon. By 1767 the surveyors had run their line 244 mi (393 km) west from the Delaware border, every fifth milestone bearing the Penn and Calvert arms. The survey was completed to the western limit of Maryland in 1773 in 1779 the line was extended to mark the southern boundary of Pennsylvania with Virginia (present-day West Virginia). Before the Civil War the term Mason-Dixon Line popularly designated the boundary dividing the slave states from the free states, and it is still used to distinguish the South from the North.

See study by E. Danson (2001).