The Mason-Dixon Line marks the boundary between Pennsylvania and Maryland. Before and during the U.S. Civil War, it became the symbolic line separating the slave states in the South from the non-slave states in the North.
A heated dispute between the Penn and Calvert families over the border between the Pennsylvania and Maryland colonies had been going on for decades before it was finally settled, and in 1763, the families commissioned Charles Mason and Jeremiah Dixon to survey and mark the boundary. In 1790, the Mason-Dixon Line took on new meaning when Pennsylvania ended slavery and made the Mason-Dixon Line the boundary between slave and free states. The Missouri Compromise in 1820 further solidified that symbolic meaning by prohibiting slavery north of the Mason-Dixon Line.