'Guinn v. United States' was a landmark Supreme Court case in Black history that was decided in 1915. The case challenged the constitutionality of the "grandfather clause," a provision in some Southern states' voting laws that exempted individuals from literacy tests and poll taxes if their grandfathers had the right to vote before the abolition of slavery.

The Supreme Court ruled in 'Guinn v. United States' that the grandfather clause was unconstitutional as a violation of the Fifteenth Amendment, which prohibits the denial of voting rights based on race. This decision marked a significant victory in the fight for voting rights for African Americans and helped to strike down similar discriminatory practices in other states.

'Guinn v. United States' was an important step towards dismantling the systemic barriers that were used to disenfranchise Black voters in the South during the Jim Crow era. This case highlighted the ongoing struggle for civil rights and equality in the United States and laid the groundwork for future legal challenges to discriminatory voting practices.