'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" laws that were used to disenfranchise Black voters in Southern states. These laws required voters to prove that their grandfathers had been eligible to vote, which effectively prevented many Black citizens from exercising their right to vote.

In a unanimous decision, the Supreme Court ruled that the Oklahoma grandfather clause violated the 15th Amendment, which prohibits racial discrimination in voting rights. This decision had significant implications for the civil rights movement and helped pave the way for future legal challenges to discriminatory voting practices.

'Guinn v. United States' was a significant victory in the fight for racial equality and voting rights, and it is remembered as an important milestone in Black history.