Missouri Compromise (1820)
"This legislation admitted Missouri as a slave state and Maine as a non-slave state at the same time, so as not to upset the balance between slave and free states in the nation. It also outlawed slavery above the 36º 30' latitude line in the remainder of the Louisiana Territory.
With the purchase of the Louisiana Territory and the application of Missouri for statehood, the long-standing balance between the number of slave states and the number of free states would be changed. Controversy arose within Congress over the issue of slavery.
Congress adopted this legislation and admitted Missouri as a slave state and Maine as a non-slave state at the same time, so that the balance between slave and free states in the nation would remain equal.
The Missouri Compromise also proposed that slavery be prohibited above the 36º 30' latitude line in the remainder of the Louisiana Territory. This provision held for 34 years, until it was repealed by the Kansas-Nebraska Act of 1854. In 1857, the Supreme Court ruled that the Missouri Compromise was unconstitutional in its Dred Scott v. Sandford decision.
The document featured here is the conference committee's report on the Missouri Compromise."
Source: https://www.archives.gov/milestone-documents/missouri-compromise#:~:text=DocumentNext%20Document-,Transcript,prohibit%20slavery%20in%20certain%20territories.