In the U.S, a grandfather clause is an exception which allows something pre-existing to remain as it is, despite a change to the contrary in the rules applied to newer situations. It is often used as the verb "to grandfather" or "grandfather in", or alternatively as "grandfathering clause". Often, such a provision is used as a compromise, to effect new rules without upsetting a well-established physical or political situation.

Some examples:

The original grandfather clauses were in laws from 1895 to 1910 in seven of the southern U.S. states as a Jim Crow law, in order to prevent blacks from voting. These laws provided that anyone allowed to vote before the American Civil War, and any of their descendants, were exempt from poll taxes levied and/or supposed "literacy" tests required at the time. Since the Fifteenth Amendment, granting former slaves the right to vote was not ratified until 1870, this disenfranchised blacks, but not whites. They were later declared unconstitutional.