An alteration or addition to a document. Although over 6,000 constitutional amendments to the US Constitution have been proposed in Congress, only 27 have been adopted, the most recent having been ratified in 1992. According to the Constitution, there are four ways in which it can be amended. An amendment can be proposed to the states either after a two-thirds vote in both houses of Congress, or by a vote in two-thirds of the state legislatures. Once it has been proposed to the states, it can be ratified either by the legislatures of three-fourths of the states or by conventions in three-fourths of the states. All 27 amendments, except the 21st Amendment, were proposed by a two-thirds majority of Congress and ratified by the legislatures of three-fourths of the states.