Mason-Dixon line

Literally, the Mason-Dixon Line (or "Mason and Dixon's Line") demarcated state boundaries between Pennsylvania, Maryland, and Delaware and parts of Virginia in colonial North America and between their successor-state members of the United States.

Due to an incorrect map, the royal charter granted Maryland the territory north of the Potomac River up to the fortieth parallel, which would put Philadelphia, the major city in Pennsylvania, within Maryland. The Calvert family, which controlled Maryland, and the Penn family, which controlled Pennsylvania, engaged two British surveyors, astronomer Charles Mason and surveyor Jeremiah Dixon, to survey what became known as the Mason-Dixon line which would form the boundary between their two colonies. This 233-mile-long line was surveyed between 1763 and 1767.

Mason and Dixon's survey also fixed the boundary between Delaware and Pennsylvania and the approximately north-south portion of the boundary between Delaware and Maryland. The Delaware-Pennsylvania boundary is a circle, and the Delaware-Maryland boundary does not run truly north-south because it was intended to bisect the Delmarva Peninsula rather than follow a meridian. However, the Maryland-Pennsylvania boundary is a true east-west line. The line also traced the border between Pennsylvania and West Virginia (originally part of Virginia).

The Mason-Dixon line thus surveyed became the symbolic boundary between the North and South, as Pennsylvania abolished slavery early while Delaware and Maryland remained slave states until the American Civil War.