Section 4(b) of the 1965 Voting Rights Act, which mandated federal oversight of changes in voting procedure in jurisdictions that have a history of using a “test or device” to impede enfranchisement. Here is one example of such a test, used in Louisiana in 1964.
http://www.slate.com/blogs/the_vault/2013/06/28/voting_rights_and_the_supreme_court_the_impossible_literacy_test_louisiana.html