In order to shrivel up the number of black votes, Southern states frequently erected a series of hurdles to be cleared before citizens could vote. These typically included certain taxes, property ownership qualifications, and educational or literacy tests.
Since the imposition of those requirements also could impact the number of poor whites voting, Southern legislatures introduced the “grandfather clause," which exempted voters from the restrictions if their grandfathers had voted. This clearly eliminated the blacks.
It was not until 1915 that the grandfather clauses were voided by the Supreme Court as a violation of the 15th Amendment.