Article V of the U.S. Constitution provides two ways for the proposal of a constitutional amendment: a two-thirds majority vote in both chambers of the U.S. Congress or by a constitutional convention called for by two-thirds of state legislatures. As of 2015, none of the 27 constitutional amendments were added through a constitutional convention.
Once Congress proposes and passes an amendment via a joint resolution, each state governor formally submits the text of that amendment to his state's legislature. A proposed amendment becomes part of the U.S. Constitution once ratified by three-fourths of the states. When the Office of the Federal Registrar verifies that it has received the required number of authentic ratification documents, the OFR drafts a formal proclamation for the Archivist of the United States to certify that the amendment is valid and now a part of the U.S. Constitution.
As of 2014, six amendments proposed by the U.S. Congress have failed to receive the required ratification by the states. The most recently failed amendment came in the form of the District of Columbia Voting Rights Amendment that would have repealed the 23rd Amendment and granted Washington D.C. full congressional representation and the ability to vote in national elections. Passed by the Congress in 1978, the amendment expired in 1985 with only 16 states having ratified the amendment.