"Guinn v. United States" was a landmark U.S. Supreme Court case in 1915 that dealt with racial discrimination in voting. The case challenged the constitutionality of the "grandfather clause" laws that were enacted in several Southern states. These laws stated that individuals could only vote if their ancestors had been eligible to vote before the abolition of slavery, effectively disenfranchising Black Americans.

The Supreme Court ruled in Guinn v. United States that the grandfather clause laws were unconstitutional because they violated the Fifteenth Amendment, which prohibits the denial of voting rights based on race. This decision was a significant victory for civil rights and marked an important step towards dismantling discriminatory voting practices.

Overall, Guinn v. United States played a crucial role in advancing the legal protection of voting rights for Black Americans and paved the way for future civil rights victories in the United States.