Constitutional amendments are rare because the process is difficult. Ratification requires two-thirds majority votes in the U.S. House and U.S. Senate and the approval of three-fourths of the state legislatures.
Two-thirds of state legislatures also can call a constitutional convention, but that has never happened. Three-fourths of the states would have to ratify any proposed amendments that emerged.
There are 27 amendments, including the original 10 that make up the Bill of Rights. The 27th Amendment, which relates to congressional pay, was ratified in 1992. However, it had been pending before the states since 1789. More recently, Congress has given the states time limits, often seven years, to ratify proposed constitutional amendments. Prior to the 27th Amendment, the Constitution hadn't been revised since 1971 when the 26th Amendment lowered the voting age to 18 in federal and state elections.