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The Mason-Dixon line began with a royal mapping blunder. Maryland’s 1632 charter to the Calvert family ran north to the 40th parallel, but when Charles II granted William Penn his colony in 1681, the boundary assumed the 40th parallel would meet a 12 mile circle around New Castle, Delaware. It does not, and the true parallel put Philadelphia inside Maryland’s grant. By the 1730s, rival tax collectors were knocking on the same farmhouse doors and Cresap’s War broke out. The Penn and Calvert heirs settled on a border 15 miles south of Philadelphia’s southernmost house and in 1763 hired astronomer Charles Mason and Quaker surveyor Jeremiah Dixon, who used a six foot zenith sector and crews of axmen at a cost of more than 3,500 pounds.

In October 1767, after 233 miles, Iroquois guides refused to go past Dunkard Creek and the survey stopped. On review, Mason and Dixon found systematic errors that the Royal Society, including Henry Cavendish, traced to the gravitational pull of the nearby Allegheny Mountains on their plumb bobs. Astronomer Royal Nevil Maskelyne sent Mason to Scotland in 1773 to find a test mountain, and the Schiehallion measurements, off by just over 11 arc seconds, led Cavendish to his torsion balance and the density of the Earth. After Pennsylvania abolished slavery in 1781, the line became the national shorthand for North and South.

• Every mile got a limestone marker shipped from England with P on one side and M on the other, and every fifth mile a crownstone bearing both family coats of arms.
• Mason and Dixon’s names were not even included in the official 1768 report of the survey.
• A crownstone was rescued from highway contractors near Elkton in 1941.
• In November 1963, John F. Kennedy dedicated a replica marker on Interstate 95, eight days before his assassination.
• Volunteers searching from 2020 to 2023 located 218 of the 226 original stones.