U.S. Constitution, Article I — The Legislative Branch, 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 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; and if Vacancies happen by Resignation, or otherwise, during the Recess of the Legislature of any State, the Executive thereof may make temporary Appointments until the next Meeting of the Legislature, which shall then fill such Vacancies.
Notes
Sets how the first Senate was divided into three classes so that one third of the seats would be chosen every second year, with terms of the first class ending after two years, the second after four and the third after six, and allows temporary appointments to fill vacancies during a legislature's recess.
Use as reference
Lexinter.net | U.S. Const. art. I, § 3, cl. 2 | https://laws.lexinter.net/constitution/us/article-1/section-3/clause-2