The Missouri Compromise of 1820 is the compromise that banned slavery above the 36-30 north latitude.
The Louisiana Purchase of 1803 nearly doubled the size of the United States. By 1820, the territory of Missouri had enough people living there to apply for statehood. However, many farmers who had moved to Missouri took their slaves with them, which led to debate. Should Missouri be admitted as a slave state, upsetting the balance in the Senate? Also, should slavery be allowed to extend into new territories as the United States expanded?
The compromise eventually admitted Missouri as a slave state and Maine as a free state to maintain the Senate balance. It also used the 36-30 north latitude line to mark the boundary between slave states and free states- at least for the time being. (See map image: Courtesy of awesomestories.com)