A grandfather clause (or grandfather policy) is a provision in which an old rule continues to apply to some existing situations while a new rule will apply to all future cases. Those exempt from the new rule are said to have grandfather rights or acquired rights, or to have been grandfathered in. Frequently, the exemption is limited; it may extend for a set time, or it may be lost under certain circumstances.
The Supreme Court case found the use of grandfather clauses unconstitutional, is the Guinn v. United States, 238 U.S. 347 (1915).
