https://www.pbs.org/wgbh/pages/frontline/shows/jefferson/mixed/onedrop.html

To be considered black in the United States not even half of one's ancestry must be African black.
But will one-fourth do, or one-eighth, or less? The nation's answer to the question 'Who is black?"

has long been that a black is any person with any known African black ancestry. This definition

reflects the long experience with slavery and later with Jim Crow segregation. In the South, it became

known as the "one-drop rule,'' meaning that a single drop of "black blood" makes a person black.

It is also known as the "one black ancestor rule," some courts have called it the "traceable amount rule,"

and anthropologists call it the "hypo-descent rule," meaning that racially mixed persons are assigned

the status of the subordinate group. This definition emerged from the American South to become the

nation's definition, generally accepted by whites and blacks. Blacks had no other choice. As we shall

see, this American cultural definition of blacks is taken for granted as readily by judges, affirmative action officers,

and black protesters as it is by Ku Klux Klansmen.