Earlier laws used fractions, such as one-eighth or one-fourth Black ancestry, to define who was Black. In the early 1900s, several Southern states tightened those rules. Virginia's Racial Integrity Act of 1924 defined a white person as someone with no trace of any non-white blood, with a limited exception for some people claiming Native ancestry. The state used it to police marriage and birth records.
The US Census counted people as "mulatto" in several censuses from 1850 to 1920 and then dropped that category. The rule meant people with mixed ancestry were assigned to the lower-status group, which kept the children of enslaved women and white men enslaved and later segregated.
Loving v. Virginia struck down the Racial Integrity Act's marriage ban in 1967. Since 2000, the Census has let people choose more than one race. The one-drop logic still shapes how many Americans see race, and it contrasts with blood quantum rules used to limit who counts as Native.