Guinn V. United States

Guinn v. United States, 238 U.S. 347 (1915), was an important United States Supreme Court decision that dealt with provisions of state constitutions that set qualifications for voters. It found grandfather clause exemptions to literacy tests to be unconstitutional. The Oklahoma Constitution, while appearing to treat all voters equally, allowed an exemption to the literacy requirement for those voters whose grandfathers had either been eligible to vote prior to January 1, 1866 or were then a resident of "some foreign nation", or were soldiers. It was an exemption that favored white voters while it disfranchised black voters, most of whose grandfathers had been slaves and therefore unable to vote before 1866.

"In 1915, in the case of Guinn v. United States, the Supreme Court declared the grandfather clauses in the Maryland and Oklahoma constitutions to be repugnant to the Fifteenth Amendment and therefore null and void." This also affected similar provisions in the constitutions of Alabama, Georgia, Louisiana, North Carolina, and Virginia. While the grandfather clause was ruled unconstitutional, state legislatures worked to develop other means of restricting voter registration. It took years for cases challenging those laws to reach the Supreme Court.