Who approves the admission of new states to the Union?

The fourth article to the Constitution determines that Congress may approve new states into the Union. However, new states are not to be formed within the jurisdiction of already-existing states or by joining two existing states or parts of existing states, without consent from the state legislature and Congress.

While the procedure to enter the Union is clearly stated in the Constitution, the procedure to leave it is not. In 1869, the Supreme Court ruled in the case of Texas v. White, that once a state joined the Union it could not leave, as the relationship is one of "perpetuity and indissolubility."