Some history about the Mason-Dixon Line:
Although the Mason-Dixon line is most commonly associated with the division between the northern and southern (free and slave, 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.
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. A decade later, the two families agreed on the compromise and set out to have the new boundary surveyed. Unfortunately, colonial surveyors were no match for the difficult job and two experts from England had to be recruited.
Charles Mason and Jeremiah Dixon arrived in Philadelphia in November 1763. Mason was an astronomer who had worked at the Royal Observatory at Greenwich and Dixon was a renowned surveyor. The two had worked together as a team prior to their assignment to the colonies.
After arriving in Philadelphia, their first task was to determine the exact absolute location of Philadelphia. From there, they began to survey the north-south line that divided the Delmarva Peninsula into the Calvert and Penn properties. Only after the Delmarva portion of the line had been completed did the duo move to mark the east-west running line between Pennsylvania and Maryland.
They precisely established the point fifteen miles south of Philadelphia and since the beginning of their line was west of Philadelphia, they had to begin their measurement to the east of the beginning of their line. They erected a limestone benchmark at their point of origin.
Over fifty years later, the boundary between the two states along the Mason-Dixon line came into the spotlight with the Missouri Compromise of 1820. This is the first time the phrase "Mason-Dixon Line" was used in history. The Compromise established a boundary between the slave states of the south and the free states of the north (however its separation of Maryland and Delaware is a bit confusing since Delaware was a slave state that stayed in the Union).
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 degrees 30 minutes North. This is todays AR-MO southern border, leading to this point.