Mason-Dixon Line

by 1779, named for Charles Mason and Jeremiah Dixon, English astronomers who surveyed (1763-7) the disputed boundary between the colonial holdings of the Penns (Pennsylvania) and the Calverts (Maryland). It became the technical boundary between "free" and "slave" states after 1804, when the last slaveholding state above it (New Jersey) passed its abolition act. As the line between "the North" and "the South" in U.S. culture, it is attested by 1834.