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The one-drop rule is a social and legal principle of racial classification that was prominent in the 20th century in the United States. It asserted that any person with even one ancestor of black ancestry ('one drop' of 'black blood') is considered black (Negro or colored in historical terms). It is an example of hypodescent, the automatic assignment of children of a mixed union between different socioeconomic or ethnic groups to the group with the lower status, regardless of proportion of ancestry in different groups.
This concept became codified into the law of some U.S. states in the early 20th century. It was associated with the principle of "invisible blackness" that developed after the long history of racial interaction in the South, which had included the hardening of slavery as a racial caste system and later segregation.