Many southern and border states devised legal barriers to circumvent the Fifteenth Amendment and prohibit black voting. These barriers included poll taxes, literacy tests, “grandfather clauses,” and the “white primary.” In 1910 Oklahoma passed a constitutional amendment, which held that only residents whose grandfathers had voted in 1865 could vote, thus disqualifying the descendants of slaves. The NAACP persuaded the U.S. attorney general to challenge the constitutionality of the “grandfather clause,” encouraged by a Maryland Circuit Court decision in 1913. Oklahoma appealed the case to the Supreme Court. Moorfield Storey was granted permission to argue the case on behalf of the NAACP. In June 1915, the Supreme Court ruled unanimously in Guinn v. United States that the “grandfather clause” was in violation of the Fifteenth Amendment.