"Guinn v. United States" was a significant U.S. Supreme Court case that was decided in 1915. The case challenged the constitutionality of the "grandfather clause," a law implemented in several Southern states that exempted individuals from literacy tests and poll taxes if their grandfathers had been eligible to vote before the abolition of slavery.

The plaintiff in the case was a Black man named William Guinn, who was disenfranchised by the grandfather clause in Oklahoma. The Supreme Court ruled that the grandfather clause violated the Fifteenth Amendment to the U.S. Constitution, which guaranteed the right to vote regardless of race. This decision marked a major victory for civil rights and voting rights activists, as it struck down a discriminatory practice that had been used to suppress Black voters in the South.

"Guinn v. United States" set an important legal precedent in the fight against voter suppression and discrimination, and it paved the way for future civil rights victories in the United States.