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Definition of MASON-DIXON LINE
the boundary line from the SW corner of Delaware N to Pennsylvania & W to approximately the SW corner of Pennsylvania; often considered the boundary between the N & S states
Originally, the boundary between Maryland and Pennsylvania. The 233-mi (375-km) line was surveyed by Charles Mason and Jeremiah Dixon in 1765–68 to define the disputed boundaries between the land grants of the Penns, proprietors of Pennsylvania, and the Baltimores, proprietors of Maryland. The term was first used in congressional debates leading to the Missouri Compromise (1820) to describe the dividing line between the slave states to its south and the free-soil states to its north. It is still used as the figurative dividing line between the North and South.