What was the border between slave and free territories west of the mississippi river?

As of 1820, the 36'30 line represented the border between slave and free territories. This line was a result of the Missouri Compromise, which was passed by the US Congress. This law made Maine a free state, Missouri a slave state, and established that any territory above 36'30 line would be free and any territory below the 36'30 line would be slave. This stayed in affect until the Compromise of 1850.