For many when you mention the Mason/Dixon line they think of it as the division between the North and the South and the Civil War. The Mason/Dixon line was nearly 100 years old when the Civil War began and was created due to the dispute between the English Colonies of Maryland and Pennsylvania.
From the time that Maryland received its charter from King Charles I on June 20, 1632, the borders were in dispute. First with the Virginia Colony and then again when William Penn was granted the Charter for Pennsylvania on February 28, 1681 by Charles II. At that time the Northern border and Southern Border of Maryland and Pennsylvania was the 40th Parallel. Penn wanted the area which became Philadelphia, which fell below the 40th parallel as part of his colony. An agreement between the two colonies was made but when Charles II gave the land area that is now Delaware to Pennsylvania it created another dispute.
It took until 1760 before the wording of the borders of the land belonging to the families of Penn’s (Pennsylvania) and the Calvert’s (Maryland) was settled and they commissioned the English team of Charles Mason and Jeremiah Dixon to survey the established boundaries.
It was on November 15, 1763 that Mason and Dixon arrived in Philadelphia to begin the task of physically marking the boundary. Nearly four years later October 9, 1767 the surveying was done.
The actual survey was marked by stones every mile (milestones) and “crownstones” every five miles. The Maryland side says (M) and Pennsylvania sides say (P). Crownstones include the two coats-of-arms. Even today nearly 250 years after their placement many of these stones are still visible. The ones on public lands are protected by iron cages.
So not only is the Mason/Dixon line the North/South border line between Pennsylvania and Maryland, it is also the East/West border line between Delaware and Maryland. At the time it was surveyed Delaware was known as the Lower Counties of Pennsylvania. It wasn’t until 1776, on June 15th, that Delaware voted to become independent from Great Britain and sever ties with the Colony of Pennsylvania.