Why were literacy tests an effective way to prevent African Americans from voting? They had to pay a tax in order to vote.
They weren't allowed to use the grandfather clause.
Most of them still spoke a language other than English.
Most of them weren't able to read and write.

Anti-literacy laws in many southern states made it illegal to teach enslaved people to read. In 1880, according to the U.S. Bureau of Census, 76 percent of southern African Americans were illiterate, a rate of 55 percent points greater than that for southern white people. In 1900, 50 percent of voting-age Black men could not read, compared to 12 percent of voting-aged white men. These disparities made literacy tests one of the most effective tools at suppressing the African American vote. The voting clerks, who were always white, could also pass or fail a person at their discretion based on race. Illiterate white people were often excluded from these literacy tests through the use of grandfather clauses, which tied their voting rights to their grandfathers' before the Civil War. Former slaves, who had no voting rights until the 15th Amendment, could obviously not benefit from this provision. The grandfather clause also applied to poll taxes, which were another measure created by white-dominated southern legislatures to suppress the Black vote.

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both include an amendment process.