"Guinn v. United States" was a landmark Supreme Court case in Black history that was decided in 1915. The case struck down the grandfather clause, a provision in many Southern states' voting laws that allowed individuals to vote only if their grandfathers had been eligible to vote before the Civil War.

The Supreme Court ruled that the grandfather clause was unconstitutional because it violated the Fifteenth Amendment, which prohibits denying the right to vote based on race. This decision was a significant victory for civil rights and helped to dismantle one of the many barriers that were erected to disenfranchise African Americans in the South during the Jim Crow era.

"Guinn v. United States" marked an important step in the fight for equal voting rights for African Americans and laid the foundation for future legal challenges to racially discriminatory voting laws.