Below the Missouri Compromise of 1820, Maine is included as a free state. Previously, the land was a piece of Massachussets. This fulfills those who opposed to the expansion of slavery along with the slave powers in congress.
As for the Northern free states, the New York Senator James Tallmadge proposed an amendment banning slavery in the Louisiana territory. Moreover, Senator Rufus King also argued congress had the power to determine whether a new state could have slavery or not.
As for the Southern Slaveholding states, Missouri is included to the union as a slave state. Even though it exists above the dividing line, the addition of Missouri as a slave state provides balance to the union in terms of free and slave states