Article V of the Constitution sets out how the Constitution itself can be changed. It begins: “The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments.” The states then decide whether to ratify them.
The first changes came quickly. Some states had ratified the Constitution “with the understanding that the amendments would be offered.” On September 25, 1789, the First Congress proposed 12 amendments. Three-fourths of the states ratified ten of them by December 15, 1791. These ten are the Bill of Rights.
Since the founding, Congress has proposed 33 amendments in all. The states have ratified 27 of them. At least 11,000 other proposals never passed Congress.