Final answer:
The grandfather clause was a discriminatory policy used to exempt white voters from literacy tests and poll taxes, effectively disenfranchising African American voters by setting a criterion that only white voters could historically meet.
Step-by-step explanation:
The purpose of a grandfather clause was to exempt certain groups of white voters from literacy tests and poll taxes that were designed to disenfranchise African American voters after the Reconstruction era. This was achieved by allowing those who had the right to vote prior to the Civil War, or their descendants, to skirt these voting prerequisites. Since African Americans were not granted the right to vote until the Fifteenth Amendment, this clause effectively barred them from voting while simultaneously allowing illiterate whites to continue voting. Literacy tests and poll taxes were other means to limit voting, but the grandfather clause specifically targeted African American voters by using historical voting rights as a criterion for exemption. The clause was eventually deemed unconstitutional, but not before it significantly reduced African American voter registration and participation in elections.