'Guinn v. United States' was a significant Supreme Court case in Black history that was decided in 1915. The case challenged the constitutionality of Oklahoma's 'grandfather clause,' a law that effectively disenfranchised Black voters by exempting individuals from literacy tests and poll taxes if their ancestors had been eligible to vote before the Civil War.

The Supreme Court ruled unanimously in 'Guinn v. United States' that the grandfather clause was unconstitutional under the Fifteenth Amendment, which prohibits states from denying citizens the right to vote based on race. This decision paved the way for greater voting rights for Black Americans and was a crucial step in the fight against discriminatory voting practices.

'Guinn v. United States' was a landmark case in the struggle for civil rights and equality, and it helped to set a precedent for future legal challenges to voting restrictions that targeted Black voters.