The 36th Parallel is a circle of latitude that is 36 degrees north of the Earth's equatorial plane. It runs across several countries, including the United States, where it passes through states like California, Arizona, and New Mexico. This parallel is significant in geography and history, often serving as a reference point for various regional boundaries. In the context of the United States, the 36th Parallel is historically notable for its role in the debate over the expansion of slavery in the 19th century. The Missouri Compromise of 1820 established this parallel as a dividing line between free and slave states, influencing the political landscape leading up to the Civil War.