What was the Missouri Compromise?
The Missouri Compromise was an agreement between the North and the South and passed by Congress in 1820 that allowed Missouri to be admitted as the 24th state in 1821. One slave state (Missouri) and one free state (Maine) were admitted to the Union, maintaining the balance. The balance of power between free and slave states in Congress was maintained to ease tensions between the North and South.
The North's attempt to force emancipation upon Missouri when it applied for admission as a slave state in 1819 rankled white southerners, and they threatened secession during the debates over the conditions under which Missouri should be granted statehood. The debates resulted in a compromise that involved the drawing of a line through the United States prohibiting slavery in future states north of the latitude 36°30′ and allowing it in future states south of that. (Missouri itself, despite lying almost entirely north of the line, was admitted as a slave state.)
This worked for about 34 years.
The Missouri Compromise was a compromise of new territory should be considered a free state or slave state. This compromise proposed whatever was north of the 36'30' line was to be a free state and whatever was south of this line was to be slave state.