- Two-thirds of both houses of Congress vote to propose an amendment, or
- Two-thirds of the state legislatures ask Congress to call a national convention to propose amendments. (This method has never been used.)
To Ratify Amendments
- Three-fourths of the state legislatures approve it, or
- Ratifying conventions in three-fourths of the states approve it. This method has been used only once — to ratify the 21st Amendment — repealing Prohibition.
The Supreme Court has stated that ratification must be within “some reasonable time after the proposal.” Beginning with the 18th amendment, it has been customary for Congress to set a definite period for ratification. In the case of the 18th, 20th, 21st, and 22nd amendments, the period set was 7 years, but there has been no determination as to just how long a “reasonable time” might extend.
Of the thousands of proposals that have been made to amend the Constitution, only 33 obtained the necessary two-thirds vote in Congress. Of those 33, only 27 amendments (including the Bill of Rights) have been ratified.
Source: Robert Longley, About.com Guide