The original grandfather clauses were contained in new state constitutions and Jim Crow laws passed from 1890 to 1910 in many of the Southern United States to prevent blacks, Mexican Americans (in Texas), and certain whites from voting. Prohibitions on freedmen's voting in place prior to 1870 were nullified by the Fifteenth Amendment.
After conservative white Democrats took control of state legislatures again after the Compromise of 1877, they began to work to restrict the ability of blacks to vote. Paramilitary groups had intimidated blacks or barred them from the polls in numerous elections prior to the Redemption. The coalition of Populists and Republicans in fusion tickets in the 1890s threatened Democratic control and increased the Democrats' desire to restrict blacks from voting. Conservative whites developed statutes and passed new constitutions creating restrictive voter registration rules. Examples included imposition of poll taxes and residency and literacy tests. An exemption to such requirements was made for all persons allowed to vote before the American Civil War, and any of their descendants. The term grandfather clause arose from the fact that the laws tied the then-current generation's voting rights to those of their grandfathers. According to Black's Law Dictionary, some Southern states adopted constitutional provisions exempting from the literacy requirements descendants of those who fought in the army or navy of the United States or of the Confederate States during a time of war.
After the U.S. Supreme Court found such provisions unconstitutional in Guinn v. United States (1915), states were forced to stop using the grandfather clauses to provide exemption to literacy tests. Without the grandfather clauses, tens of thousands of poor Southern whites were disfranchised in the early 20th century. As decades passed, Southern states tended to expand the franchise for poor whites, but most blacks could not vote until after passage of the 1965 Voting Rights Act. Ratification in 1964 of the Twenty-fourth Amendment to the United States Constitution prohibited the use of poll taxes in federal elections, but some states continued to use them in state elections.
The 1965 Voting Rights Act had provisions to protect voter registration and access to elections, with federal enforcement and supervision where necessary. In 1966, the Supreme Court ruled in Harper v. Virginia Board of Elections that poll taxes could not be used in any elections. The franchise was thus secured for most citizens, and voter registration and turnout climbed dramatically in Southern states. Once again, African Americans were able to participate fully in political life and began to win elective office at many levels.
http://en.wikipedia.org/wiki/Grandfather_clause