Amending the U.S. Constitution is, by design, a very difficult process. Since the adoption of the Bill of Rights – the first 10 amendments to the Constitution, ratified in 1791 – only 17 amendments have cleared the hurdles necessary to be codified in the nation's founding document, the last of which was ratified in 1992. There are four ways to amend the Constitution, though only two have ever been used.

The Most Common Method

Nearly every constitutional amendment – 26 out of 27, in fact – have taken the same course: The House of Representatives and Senate both vote on the proposed amendment; the Constitution requires that for the proposed amendment to pass, each house of Congress must pass it with a two-thirds majority. If that happens, the amendment is then sent to state legislatures for their approval. Here again, another, even larger supermajority is required: Three-fourths of all state legislatures must vote to approve the proposed amendment before it is considered ratified and added to the Constitution. This means that only 13 states can block a proposed amendment from being ratified.

The Convention of States

The first two amendment processes have never been undertaken, and for that reason it’s unclear how they would actually function. However, the Constitution allows for two-thirds of the states, through their legislatures, to call for a national convention to amend the Constitution. This Constitutional Convention could then propose new amendments. Under the first method, these amendments would then have to be approved by three-fourths of state legislatures (i.e., 38 out of the current 50). The second method is similar to the first, except that instead of the amendment being sent to state legislatures for ratification, it would be sent to conventions within the states. Three-fourths of these conventions would have to sign off before the amendment would be ratified.

The 21st Amendment

The 21st Amendment – which repealed the 18th Amendment’s prohibition on alcohol – is the only amendment to employ this final method. Two-thirds of both houses of Congress voted to send this amendment to the states for ratification, but instead of insisting that three-fourths of state legislatures approve the amendment, Congress specified that “this article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by conventions in the several States … within seven years.” Congress did so because proponents believed elected delegates to these conventions were more likely to reflect public opinion on Prohibition. Within 10 months of Congress’s passage, the 21st Amendment was approved.

The 27th Amendment

The latest amendment to the Constitution, the 27th, ratified in 1992, took by far the longest period of time between proposal and ratification – 74,003 days, to be exact, or more than 200 years. This amendment, which prevents members of Congress from voting to alter their pay during a congressional session, was originally proposed by James Madison in 1789. By 1791 it had been approved by a half-dozen states. But then, for nearly two centuries, it lay dormant. As Americans’ frustrations with congressional pay raises grew in the late 20th century, Madison’s proposal – which unlike many more recent amendments did not have a time restriction on its adoption – gained steam, and was finally ratified in 1992.