The Twenty-seventh Amendment to the United States Constitution (Amendment XXVII) has to do with the salaries paid to members of the United States Congress.  

Congress is made of two "Houses," and the members of each House serve different terms (amounts of time in office).  Members of the United States Senate serve six-year terms, and members of the United States House of Representatives serve two years each.  The Twenty-seventh Amendment says that no law can change Congresspeople's salaries until a new two-year term for Representatives starts.

As of 2022, the Twenty-seventh amendment is the last amendment that has been added to the Constitution.  It took longer for the states to ratify this amendment than any other in history.  The 1st United States Congress sent the suggested amendment to the states for their approval on September 25, 1789.  It was not until May 7, 1992, that enough states ratified the amendment for it to be added to the Constitution.  The ratification process had taken 202 years, 7 months, and 12 days  the longest in United States history.

Text
When Congress first proposed the amendment in 1789, this is the text they suggested:

History
On September 25, 1789, the first United States Congress gave a group of 12 amendments to the states for their approval.  Written by James Madison, these were the first 12 Constitutional amendments ever written.  One of them was an amendment about salaries for Members of Congress.  Madison wanted to protect against Congresspeople being able to give themselves raises whenever they wanted to, while they were in the middle of their terms and the people could not vote them out.

In 1791, the states ratified ten of the 12 suggested amendments.  These ten amendments became the Bill of Rights.  However, the states did not ratify the other two amendments out of the 12  including the one on limiting pay for Congresspeople.

Proposal and ratification

Proposal by Congress 
James Madison first proposed this amendment in the United States House of Representatives on June 8, 1789.  On August 24, 1789, the House passed the suggested amendment and sixteen others. The proposals went next to the United States Senate, which made 26 different changes to the House's suggested amendments.  After taking out some parts of the House amendments, and combining others, the Senate approved a package of twelve suggested amendments on September 9, 1789.

Now a committee of House and Senate members had to meet to figure out a compromise and come up with amendments that they could all agree on.  After three days, they had come up with 12 suggested amendments, which they brought back to the full House and Senate.  That same day  September 24, 1789  the House agreed on the 12 proposed amendments.  The next day, the Senate agreed too.  The 12 amendments included the one on Congressional pay.

Ratification by the states

Congress sent the package of 12 amendments to the states on September 25, 1789.  For these amendments to be added to the Constitution, three-fourths of the state legislatures would have to ratify (approve) the amendments.  At the time, there were only 11 states (North Carolina and Rhode Island had not ratified the Constitution yet).  This meant just nine states would have to approve the amendments to add them to the Constitution.

On December 15, 1791, the states ratified ten of the twelve suggested amendments.  These got added to the Constitution together as the Bill of Rights.  However, at this time, only six states had ratified the Congressional pay amendment.  

Over the next 186 years, only two more states ratified the Amendment.  Meanwhile, many more states joined the United States.  This meant that a larger number of states would need to ratify the amendment to get it added to the Constitution, since three-fourths of the state legislatures needed to ratify it, and there were more and more states as time went on.

The states ratified the Amendment in this order:

Notes

Reference

27
1992 in the United States
1990s in law
1780s in the United States
1789