Mason and Dixon's Line, or Mason-Dixon Line, in United States history, the symbolic dividing line between the North and the South. It takes its name from two Englishmen, Charles Mason and Jeremiah Dixon. During 1763–67, they surveyed the southern boundary line of Pennsylvania, east to west along 39° 43' 17.6", which separated it from Maryland and the part of Virginia that is now West Virginia. In 1768 they extended the line southward, establishing the boundary between Maryland and Delaware (then a part of Pennsylvania).

Pennsylvania and Maryland had long argued over territorial claims before agreeing in 1760 to have a survey made of the boundary line. Mason and Dixon, beginning at the eastern border of Maryland, marked the line westward 233 miles (375 km) to Dunkard Creek, unwittingly going about 30 miles (48 km) past Maryland's western border. Later the line was completed to Pennsylvania's western border. In 1784 Virginia agreed to accept the line as its border with Pennsylvania.

The eastern portion of Mason and Dixon's Line was marked by milestones. On every fifth stone were carved the coats of arms of the Penn and Calvert families, proprietors respectively of Pennsylvania and Maryland. Resurveys during 1849–50 and 1901–03 of the Pennsylvania boundary and 1961–62 of the Maryland-Delaware line revealed no errors of importance.

Mason and Dixon's Line came to be thought of as a North-South dividing line because Pennsylvania was a free state and Maryland and Virginia were slave states. Northerners and Southerners are still sometimes said to come from “above” or “below” the line.