Twenty-sixth Amendment to the United States Constitution
The Court ruled that Congress could set a voting age for federal elections, but not state or local elections.  This meant that for state and local elections, states could keep the voting age at 21 if they wanted to.  However, they would have to keep two different voting registers (lists of people who had signed up to vote): one for federal elections, which would include people as young as 18, and another for state and local elections, which would only include people over age 20.