The U.S. Constitution provides a process for changing the document, allowing flexibility in what's often called the Law of the Land.

It is a process that sets high barriers to change.

In general, an amendment must be approved by both the U.S. House and Senate by a two-thirds majority. It is then up to the states to ratify the amendment. If three-fourth of the states, or 38 of 50, approve the language -- in either state legislatures or special conventions -- the amendment becomes part of the Constitution.

There have been 27 amendments to the Constitution, including the first 10, the Bill of Rights, which were ratified on Dec. 15, 1791.

The last amendment to be ratified was originally proposed as part of the Bill of Rights. However, it did not have an expiration date for ratification, and was ocassionally revived. It was eventually ratified in 1992. The 27th Amendment prevents pay changes for Congress from going into effect before an election.

Other amendments have dealt with social issues such as slavery, prohibiting the drinking of alcohol (later repealed) and lowering the voting age to 18.

Read the Constitution and learn more about the amendments at USConstitution.net.