amendment, in law, alteration of the provisions of a legal document. The term usually refers to the alteration of a statute or a constitution, but it is also applied in parliamentary law to proposed changes to a bill or motion under consideration, and in judicial procedure to the correction of errors. A statute may be amended by the passage of an act that is identified specifically as an amendment to it or by a new statute that renders some of its provisions nugatory. Written constitutions, however, for the most part must be amended by an exactly prescribed procedure. The Constitution of the United States, as provided in Article 5, may be amended when two thirds of each house of Congress approves a proposed amendment (approval by the president is not required), and three fourths of the states thereafter ratify it, sometimes within a set period. Congress decides whether state ratification shall be by vote of the legislatures or by popularly elected conventions. Only in the case of the Twenty-first Amendment (repealing prohibition) has the convention system been used. In many U.S. states, a proposed amendment to the state constitution must be submitted to the voters in a referendum.