The Mason-Dixon Line is not just one line: it is three. It determines the southern border of Delaware, its western border with Maryland, and the border between Maryland and Pennsylvania. It is this last line, the Pennsylvania-Maryland divide, that is most often meant by the term.
Two English kings caused the confusion that led to surveys of the Delaware, Maryland, and Pennsylvania borders. In 1632, King Charles I gave a favorite subject, Cecilius Calvert, the land now known as Maryland, named after the English Queen Henrietta Maria. In 1681, King Charles II gave his chum William Penn the colony of Delaware and the land Penn named Pennsylvania. In theory, these lands were both well defined, as Maryland’s northern border was supposed to meet Pennsylvania’s southern border at forty degrees latitude. In fact, the kings never measured the properties, and Calvert and Penn were overlapping each others’ territories without realizing it.
As sea trade began to prosper between Europe and America, European ship crews brought advanced surveying equipment to the colonies, where they discovered the problem with the Maryland-Pennsylvania boundary and informed Penn. In response, Penn tried to tax some of Calvert’s border residents, and Calvert retaliated by making claims to taxes from Penn’s settlers. The colonists refused to pay two taxes at once—in some cases, they refused to pay at all—and the ensuing conflict caused a number of fatalities.
The king intervened in 1750 and ordered a settlement that turned out to provide only a partial resolution. The first surveyors identified a midpoint between the Atlantic Ocean and Chesapeake Bay, but the Calvert and Penn families also wanted a survey of a line running north from that point. A new team began the project in 1760, but after struggling to make progress, the Calvert and Penn parties decided to contact the British Royal Observatory for a referral. The Observatory recommended two English experts, Charles Mason and Jeremiah Dixon, for the job. By 1763, the Penn and Calvert grandchildren had a contract with Mason and Dixon for the survey.
Mason and Dixon had already garnered prestige in the European community for their astrological observations and mathematical sophistication, and they applied the same methods to the uncharted east coast of North America. They met new challenges in the form of swamps, difficult weather, and Native American tribes unhappy with trespassers, but finished the survey in 1768 and returned to England. Mason and Dixon left behind a landmark achievement in geodesy, the field of mathematics that concerns calculating points on the earth’s surface.
Eight years after Mason and Dixon settled the Calverts’ and Penns’ dispute for good, the American Revolution took hold, stripping both families of their land in the former colonies, now states in the new United States of America. The Mason-Dixon Line, as it came to be known, endured far beyond British ownership, assuming immense symbolic importance during the Missouri Compromise negotiations in 1820. According to the Compromise, all states south of thirty-six degrees thirty minutes in latitude could continue their slave trade, while those north of the line would be free states. Missouri was the noted exception.
As Congress deliberated on the matter, Thomas Jefferson provided insight on “the Missouri question” in a letter to his friend John Holmes. Jefferson predicted the tension the Mason-Dixon Line would come to represent, commenting, “A geographical line, coinciding with a marked principle, moral and political, once conceived and held up to the angry passions of men, will never be obliterated; and every new irritation will mark it deeper and deeper.” He was right. The Mason-Dixon Line still marks a division, political, social, and cultural, between North and South.