The second clause of Article I, Section 3 of the U.S. Constitution addressed the incoming First Congress as follows:
Immediately after they shall be assembled in Consequence of the first Election, they shall be divided as equally as may be into three Classes. The Seats of the Senators of the first Class shall be vacated at the Expiration of the second Year, of the second Class at the Expiration of the fourth Year, and of the third Class at the Expiration of the sixth Year, so that one third may be chosen every second Year....
This original instruction is followed today with roughly one-third of the Senate standing for reelection every two years.
Click on the following links to find a listing of the Senators, by class, who have served in each of the states: