Most Americans know the Mason-Dixon Line as the divider between North and South; freedom and slavery. But the line's origins have nothing to do with slavery and actually predate the United States. The line is, in fact, the result of a bloody land dispute between proprietors of Pennsylvania and Maryland when the country was just a collection of British colonies.
Due to imprecise and confusing land grants, the Penn family (the owners of Pennsylvania) and the Calvert family (who owned Maryland) couldn't agree on the boundaries between the two colonies. The dispute was submitted to the English court of chancery in 1735. The Court Chancery in England ruled that the latitude 39°43' north (15 miles south of Philadelphia) should serve as the Pennsylvania / Maryland border (which runs east-west). A compromise between two families in 1760 resulted in the appointment of the English team of Charles Mason, a mathematician and astronomer, and Jeremiah Dixon, a mathematician and land surveyor, to survey the boundary. By 1767 the surveyors had run their line 244 miles west from the Delaware border, with every fifth milestone bearing the Penn and Calvert arms. The survey was completed to the western limit of Maryland in 1773; in 1779 the line was extended to mark the southern boundary of Pennsylvania with Virginia (present-day West Virginia).
Mason-Dixon Line, boundary between Pennsylvania and Maryland (running between lat. 39°43'26.3"N and lat. 39°43'17.6"N), surveyed by the English team of Charles Mason, a mathematician and astronomer, and Jeremiah Dixon, a mathematician and land surveyor, between 1763 and 1767.
It wasn't until the Missouri Compromise of 1820 that the boundary took on the role of front line in the war on slavery. The Compromise established a boundary between the slave states of the south and the free states of the north. This boundary became referred to as the Mason-Dixon line because it began in the east along the Mason-Dixon line and headed westward to the Ohio River and along the Ohio to its mouth at the Mississippi River and then west along 36° 30' North.