Constitutional amendment

An amendment is a formal revision or addition to the US Constitution. As per Article V of the Constitution, there are several methods to propose an amendment. Once an amendment is proposed, it requires the approval of ¾ of the states to be ratified. 

Traditionally, states are free to ratify proposed amendments at their leisure for an indefinite period of time. For example, the 27th Amendment was not ratified by the necessary number of states until over 200 years after it was proposed. Modern amendment proposals tend to include a 7-year time frame for states to ratify an amendment. For this reason, the Equal Rights Amendment has not become law despite achieving ratification by ¾ of the states. 

Currently, there are 27 amendments to the United States Constitution. The first 10 amendments are known as the Bill of Rights.