grandfather clause

noun

1. : a clause creating an exemption (as from a law or regulation) based on circumstances previously existing; specifically : a provision inserted in the constitutions of some southern states after the Civil War requiring high standards of literacy and substantial property qualifications of voters except for descendants of men voting before 1867 : a provision inserted in the constitutions of some southern states after the Civil War requiring high standards of literacy and substantial property qualifications of voters except for descendants of men voting before 1867