"Guinn v. United States" was a significant U.S. Supreme Court case in 1915 that challenged the constitutionality of the "grandfather clause" as a method to disenfranchise Black voters. The grandfather clause was a Jim Crow law that exempted individuals from literacy tests and poll taxes for voting if their grandfathers were eligible to vote before the Civil War. This effectively allowed illiterate white voters to bypass literacy requirements, while disenfranchising many Black voters whose ancestors were enslaved and unable to vote.

In the case, the Supreme Court declared the grandfather clause unconstitutional under the 15th Amendment, which prohibits states from denying the right to vote based on race. This decision was a significant victory for civil rights and voting rights advocates, as it struck down a discriminatory practice that had been widely used to suppress Black voter turnout in the South. The ruling in "Guinn v. United States" helped pave the way for future legal challenges to discriminatory voting laws and contributed to the ongoing struggle for voting rights and racial equality in the United States.