How the Senate Works
The United States Senate has 100 members, two for each state. Obviously, the size of the body has increased over the years, but each state has been awarded two Senators since the ratification of the Constitution. The differentiation between Senate and House apportionment is the result of a compromise between big states and small states whose representatives were each weary of the other.
There are three classes of Senators - one class is elected every two years for a six year team each. Class 1 has 33 Senators, Class 2 has 33 and Class 3 has 34. This is outlined in the Constitution Article 1, section 3, clause 2 - "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 U.S. Senate history page notes that even this division was up for debate -
At the 1787 Constitutional Convention, delegates linked the Senate class measure to the debate on term lengths. On June 25, Nathaniel Gorham suggested a four-year Senate term with one-fourth of the senators elected each year. Edmund Randolph supported staggered rotation in the Senate, but wanted a seven-year term “to go out in fixt proportion.” Hugh Williamson countered that six-year terms were more easily divisible into equal election cycles than seven-year terms. The following day, Gorham called for a six-year term, “one third of the members to go out every second year.” Delegates considered a nine-year term, then passed the six-year, three-class Senate clause by a vote of 7 to 4.
Ohio, which entered the union in 1803, had its Senators entered into Classes 1 and 3.
Senators were elected by State Legislatures until 1914 when direct election of Senators started.