In 1820, an agreement was reached between pro-slavery and anti-slavery parties to allow slavery in certain parts of western territory, namely Missouri. This decision is known as The Missouri Compromise, or the Compromise of 1820. Specifically, slavery would be prohibited in all new states north of the 36 degree 30' line, except for Missouri. This decision was highly controversial. As Thomas Jefferson wrote in a letter to John Holmes: "A geographical line, coinciding with a marked principle, moral and political, once conceived and held up to the angry passions of men, will never be obliterated; and every new irritation will mark it deeper and deeper." Of course hindsight shows us just how deeply the line would be marked. You can actually visit the line if you travel to Branson Missouri and ask around for the location.