The Thirty-First Parallel North is a circle of latitude that is 31 degrees north of the Earth's equatorial plane. It runs through several countries, including the United States, Mexico, and parts of North Africa and Asia. This parallel is significant in geography and is often used as a reference point for mapping and navigation. In the United States, the Thirty-First Parallel North passes through states like California, Arizona, and Texas. It is also notable for its historical significance, as it has been used in various treaties and boundary agreements, particularly in the context of U.S. territorial expansion.