"Guinn v. United States" was a significant U.S. Supreme Court case that was decided in 1915. The case dealt with the issue of racial discrimination in voting practices, particularly the use of "grandfather clauses" as a means to disenfranchise Black voters.

At the time, several Southern states had implemented grandfather clauses as part of their voter registration laws. These clauses exempted individuals from literacy tests, poll taxes, and other voting requirements if their grandfathers had been eligible to vote before the abolition of slavery. The intention behind these clauses was to allow illiterate or poor white voters to continue voting while disenfranchising Black voters whose ancestors had been enslaved.

In the Guinn v. United States case, the Supreme Court ruled that grandfather clauses (as well as other similar practices that served to disenfranchise Black voters) were unconstitutional under the Fifteenth Amendment, which prohibits the denial of voting rights based on race. This decision was a significant step towards protecting the voting rights of African Americans and challenging discriminatory voting practices in the United States.