The privileges and immunities clause (U.S. Const., art. IV, sec. 2, cl. 1) generally prohibits states from discriminating against those who aren't state citizens, or from favoring their own citizens over citizens of other states, when it comes to the exercise of certain fundamental rights. The privileges and immunities clause requires each state to treat citizens of other states who are exercising certain fundamental rights as it treats its own citizens, unless the state can prove that different treatment is substantially related to a substantial state interest.