Ways to Amend the Constitution
Under Article V of the Constitution, there are two ways to propose amendments to the Constitution and two ways to be ratified by the states.
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, only 33 obtained the necessary two-thirds vote in Congress. Of the 33, 27 amendments have passed. The 6 unratified amendments may be found in the U.S. Constitution Amended, Unratified Amendments, Analytical Index [PDF, 390k] [TEXT, 295k] on GPO Access.