Which U.S. Supreme Court case established the 'one person, one vote' principle?

Reynolds v. Sims established the “one person, one vote” principle for state legislative districts. In its 1964 decision, the Supreme Court ruled that both chambers of Alabama’s legislature had to be apportioned according to population so that voters’ representation would be substantially equal.

The case arose because Alabama’s legislative districts had not been regularly reapportioned, leaving people in populous areas with far less voting power than residents of sparsely populated counties. Chief Justice Earl Warren’s majority opinion held that the Equal Protection Clause of the Fourteenth Amendment protects voters against such severe population disparities.

Several neighboring cases are easy to confuse with Reynolds. Baker v. Carr, decided in 1962, made legislative-apportionment disputes reviewable in federal court. Gray v. Sanders applied equal-population reasoning to statewide elections, and Wesberry v. Sanders addressed congressional districts. Reynolds is the canonical answer for equal representation in state legislatures.