"Guinn v. United States" was a landmark Supreme Court case that was decided in 1915. This case was significant in Black history as it struck down the "grandfather clause" which had been used in several Southern states to disenfranchise Black voters.

The grandfather clause was a law that allowed individuals to vote only if their grandfathers had been eligible to vote before the Civil War. Since former slaves were not allowed to vote before the war, this effectively excluded most Black Americans from voting.

In the Guinn case, the Supreme Court ruled that the grandfather clause was unconstitutional because it violated the Fifteenth Amendment, which guaranteed the right to vote regardless of race. This decision was a major victory for civil rights and helped pave the way for future voting rights advancements for Black Americans.