in 1781, pennsylvania abolished slavery. the original line became the rough dividing line between southern slave states and northern free states. the exception was deleware which remained a slave state up to the american civil war. fifty years later the missouri compromise of 1820 established a line extending the mason–dixon line westward to the ohio river. then following the border of ohio to the mouth of the river at the mississippi river, then westward along 36 degrees 30 minutes north. the line began as a way to end a border dispute and became best known as the line separating slave and free states.