Although the Mason-Dixon line is most commonly associated with the division between the northern and southern (free and pro-slavery, respectively) states during the 1800s and American Civil War-era, the line was delineated in the mid-1700s to settle a property dispute. The two surveyors who mapped the line, Charles Mason and Jeremiah Dixon, will always be known for their famous boundary.
Map indicating the Mason Dixon line.
Alvin Jewett Johnson / Public domain / Wikimedia Commons
Calvert vs. Penn
In 1632, King Charles I of England gave the first Lord Baltimore, George Calvert, the colony of Maryland. Fifty years later, in 1682, King Charles II gave William Penn the territory to the north, which later became Pennsylvania. A year later, Charles II gave Penn land on the Delmarva Peninsula (the peninsula that includes the eastern portion of modern Maryland and all of Delaware).
The description of the boundaries in the grants to Calvert and Penn did not match and there was a great deal of confusion as to where the boundary (supposedly along 40 degrees north) lay. The Calvert and Penn families took the matter to the British court and England's chief justice declared in 1750 that the boundary between southern Pennsylvania and northern Maryland should lie 15 miles south of Philadelphia.
