Amending the U.S. Constitution proceeds in two stages. At the first or proposal, stage, an amendment may be proposed by a two-vote of both houses of Congress or by a two-thirds vote of a national convention called by Congress on the request of two-thirds of state legislatures for the express purpose of proposing changes. A national convention has never been used. At the second or ratification stage, a proposal must be passed by three-quarters of state legislatures or by special ratification conventions held in at least three-quarters of the states. Special ratification conventions have been used only once to pass the twenty-first amendment repealing Prohibition.