Article Four of the United States Constitution
The First Clause of Section Three grants to Congress the authority to admit new states into the Union. Since the establishment of the United States in 1776, the number of states has expanded from the original 13 to 50. It also forbids the creation of new states from parts of existing states without the consent of both the affected states and Congress. This latter provision was designed to gave Eastern states that still had claims to Western lands (e.g., Virginia and North Carolina) to have a veto over whether their western counties (which eventually became Kentucky and Tennessee) could become states. It would later be applied with regard to the formation of Maine (from Massachusetts) and West Virginia (from Virginia).