Mason-Dixon Line: A boundary originally surveyed in the 1760s that separated Pennsylvania and Maryland, symbolically dividing free states (north) from slave states (south).
36°30' Parallel: A line of latitude used as part of the Missouri Compromise in 1820 to define which territories would allow slavery (south of the line) and which would be free (north of the line).
Kansas-Nebraska Act: An 1854 law that allowed settlers in Kansas and Nebraska territories to decide whether they wanted slavery through popular sovereignty, effectively erasing previous lines between slave and free states.