The Mason-Dixon Line has two definitions. One relates to the surveying of land by Charles Mason and Jeremiah Dixon from 1763-1767. The other definition refers to separation between US states by states that allowed slavery, and states which did not, or Union and Confederate States. This more common usage occurred during the Civil War.
The initial purpose of this line as drawn by Mason and Dixon was to settle disputes between people who owned land in Maryland and Pennsylvania. The boundaries of this land were in dispute and the families owning the land, the Calverts and the Penns, were contentious in regard to these borders.
Initially, The Mason-Dixon Line ran east-west through the Southern border of Pennsylvania, and north-south between the borders of Maryland and Delaware. The lines were marked by stones each at the end of each mile. Every five miles, a large crownstone was also used.
Dispute between which areas of property belonging to Delaware and Pennsylvania resulted in a 1732 agreement of specific borders. However, arguments about colony borders still existed. Finally in 1760, the King of England forced the people of both areas to accept the agreement made in 1732, and Mason and Dixon, two astronomers, were commissioned to create the line.
Later, the Mason-Dixon Line was defined as the separation between states that had seceded from the Union. The actual line, which was really symbolic in purpose, is slightly harder to define. The border states like Missouri, Kentucky, Maryland and West Virginia are sometimes considered as below the line. On other maps, the border states are north of the line.
The Mason-Dixon Line extends to Texas, which is often considered the most western of the southern states. In many ways, continued discrimination against Blacks in the southern states was seen as continued observation of the Mason-Dixon line. Though slavery was outlawed at the close of the Civil War, the Mason-Dixon line was thought of as a symbolic separation between states that continued discrimination against Blacks and those which did not to the same extent. However, it should be noted that states north of the line often proved just as terrible in their treatment and assessment of the rights of Blacks.