- the boundary between Pennsylvania and Maryland, partly surveyed by Charles Mason and Jeremiah Dixon between 1763 and 1767, popularly considered before the end of slavery as a line of demarcation between Free States and Slave States.
- the state boundary between Maryland and Pennsylvania: surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon; popularly regarded as the dividing line between North and South, esp between the free and the slave states before the American Civil War
