The influence of the work performed by these respected British surveyors and astronomers was not fully realized until the line they surveyed was woven into the history of the Civil War nearly 100 years later. The "Mason-Dixon Line" became an icon of American culture when it was recognized as the line that divided the "free states" from the "slave states" during the debates in Congress over the Missouri Compromise in 1820.

The conflict of land ownership which predicated the request for their services dated back to the colonization of Pennsylvania by Englishman William Penn in 1681. The Maryland charter dated from 1632, and granted the province of Maryland to Cecilius Calvert, second Lord Baltimore, describing his land as:

"...all that part of a Peninfula, lying in the parts of America, betweene the Ocean on the Eaft, and the Bay of Chefopeack on the Weft, and divided from the other part thereof, by a right line drawne from the Promontory or Cape of Land called Watkins Point (fituate in the forefaid Bay, neere the river of Wigh-- on the Weft, unto the maine Ocean on the Eaft; and betweene that bound on the South, unto that part of Delaware Bay on the North, which lieth under the fortieth degree of Northerly Latitude...

The Pennsylvania charter described Penn's southeastern and southern boundaries as:

"...a circle drawn at twelve miles distance from New Castle, northward and westward unto the beginning of the fortieth degree of northern latitude, and then by a straight line westward."

The true position of this "fortieth degree of northern latitude" described in both charters became a sticking point for the Penn and Calvert families for many generations. The dispute was submitted to the English court of chancery in 1735. A compromise between the families in 1760 resulted in the appointment of surveyors Charles Mason and Jeremiah Dixon who began their survey in 1763. By 1767 the surveyors had run their line 244 miles west from the circular boundary of Delaware (at the actual latitude of approximately 39°43'N), with every fifth milestone bearing images of the Penn and Calvert arms, when Charles Mason wrote:

"...we cross'd a Warrior Path...there we were informed by the Chief of our Indians that he was come to the Extent of his Commission...and that he would not proceed one Step further."

The survey was later completed by Andrew Ellicott and David Rittenhouse to the true southwest corner of Pennsylvania.

Before the Civil War, the term "Mason-Dixon Line" popularly designated the boundary dividing the slave states from the free states, although West Virginia (lying south of the line) had appealed for statehood as a "free" state during the war, and Maryland itself sided with the north. To this day, the "Mason-Dixon Line" is still referred to as the separation of the South from the North.