Article IV, Section 3 of the Constitution requires state legislatures to consent before Congress can form new states from their territory: "No new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress."
The Framers designed this provision to give eastern states with western land claims veto power over whether their territories could become separate states. Four states—Vermont, Kentucky, Maine, and West Virginia—were created from land legally held by existing states through this consent process. Virginia's legislature consented to Kentucky's admission from its western counties in 1792, North Carolina approved Tennessee, and Massachusetts approved Maine. West Virginia presented a unique case during the Civil War when Virginia's Restored Government, recognized by the Union, consented to the western counties separating.
Beyond requiring affected state consent and congressional approval, the Admissions Clause leaves procedural details of the admission process to Congress, which has admitted 37 states since the original 13 ratified the Constitution.