'Guinn v. United States' was a landmark U.S. Supreme Court case in Black history that was decided in 1915. This case struck down the 'grandfather clause' as a voting restriction in Oklahoma and Maryland. The 'grandfather clause' was a racist provision that exempted men from strict voting requirements if they or their ancestors had been eligible to vote before the Civil War - effectively disenfranchising Black voters who had been enslaved and their descendants.

The Supreme Court's decision in 'Guinn v. United States' was a significant victory for civil rights and marked an important step in the fight against voter suppression and discrimination. It highlighted the importance of protecting voting rights for all citizens, regardless of race or background.