Southern states sometimes used a grandfather clause to allow them to

Southern states sometimes used a grandfather clause to hinder African-Americans from voting and let the whites to vote.

Explanation:

The Grandfather clause is defined as "a clause exempting certain pre-existing classes of people or things from the requirements of a piece of legislation"

It enacted for voting purpose on Tuesday, 2nd of August, 1898 as a legal or constitutional mechanism passed by seven Southern states during reconstruction to deny franchise (the right to vote in political elections) to black Americans.