Southern states have used a variety of methods to disenfranchise African American voters. Empirical data on the effectiveness of these measures is rare. We present a unique data source from Louisiana that allows us to empirically document voter registration rates from the end of Reconstruction to the present. Using basic time series data, we document how voter registration rates changed over time in response to state restrictions. We then conduct a second analysis, which focuses on Louisiana’s use of the Understanding Clause to reduce voter registration among Blacks. We show that in parishes that used the Understanding Clause, Black registration rates dropped by nearly 30 percentage points, with little effect on white registration. The findings of this paper have important implications for understanding the potential for discrimination in the enforcement of modern, ostensibly nonracial, voter eligibility requirements, such as voter ID laws, which grant substantial discretion to local officials in determining voter eligibility.