grandfather clause

     A grandfather clause was a ploy used in the South to keep the recently emancipated blacks from voting, during the era of reconstruction after the Civil War. After the US Constitution was amended to give black males the right to vote, some Southern states passed "grandfather clause" laws.  Under these laws, you couldn't vote unless your grandfather could vote--they effectively disenfranchised blacks in the South. This is illegal now. What is more recently called "grandfathering" something means to make it retroactive--as if it had begun at an earlier date.