Question

Which Supreme Court case found the use of grandfather clauses unconstitutional?

Answer 1

The Supreme Court case that found the use of the grandfather clause unconstitutional is the Guinn v. United States, 238 U.S. 347 (1915). In its decision, the Supreme Court ruled that  the grandfather clause exemptions to literacy tests were unconstitutional. It declared the grandfather clauses as repugnant to the Fifteenth Amendment and therefore null and void.

Answer 2

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).