Mason-Dixon lineMa·son-Dix·on Line
boundary line between Pa. & Md., regarded, before the Civil War, as separating the free states from the slave states or, now, the North from the South
also Mason and Dixon's line
Origin of Mason-Dixon lineafter C. Mason and J. Dixon, who surveyed it, 1763-67
A line marking the boundary between the colonies of Pennsylvania and Maryland, established between 1763 and 1767 by the British surveyors Charles Mason (1730-1787) and Jeremiah Dixon (1733-1779). It was regarded as the division between free and slave states before the Civil War.
The original line extended from the southeast corner of the colony of Pennsylvania 233 miles west and 82 miles south.