The Constitution provides that senators are elected to six-year terms, and that one-third of the senators are to be elected every two years "divided as equally as may be into three Classes." (Article I,
Section 3, clause 2.) A class is the approximately one-third of the Senate elected in the same general election
held in even years, which term begins the following year (e.g., the
Senate term for a senator elected in 2016 begins in 2017 (in the
first session of the 114th
Congress) and ends in 2023 (after the end of the
117th Congress). Senate terms
of Senators appointed or elected to fill vacant Senate seats generally
last through the end of the term of the senator being replaced. The
replacement procedure is outlined by the
17th Amendment, and varies by state.
Class I terms run from the beginning of the 110th Congress on January 3, 2007, to the end of the 112th Congress on January 3, 2013. Senators in Class I were elected to office in the November 2006 general election, unless they took their seat through appointment or special election.
Class II terms run from the beginning of the 111th Congress on January 3, 2009, to the end of the 113th Congress on January 3, 2015. Senators in Class II were elected to office in the November 2008 general election, unless they took their seat through appointment or special election.
Class III terms run from the beginning of the 112th Congress on January 3, 2011, to the end of the 114th Congress on January 3, 2017. Senators in Class III were elected to office in the November 2010 general election, unless they took their seat through appointment or special election.