What drew a line north of which any new states would be free states and the south of which any new states would be slave states?

The line that established the division between free states and slave states was the Missouri Compromise of 1820, which drew a latitude line at 36°30′ north. States north of this line, except for Missouri, were designated as free states, while those to the south were allowed to permit slavery. This compromise aimed to maintain the balance of power between free and slave states in the U.S. Congress. However, it was eventually rendered ineffective by later legislation, such as the Kansas-Nebraska Act of 1854.