The one-drop rule was the classification of anyone with even one African ancestor, or one drop of African blood, as black. This rule began as a common social norm in the early 19th century and was ultimately enshrined in American miscegenation laws starting in 1924.
In colonial America, it was not uncommon for free blacks to intermarry with white European immigrants, resulting in several melungeon, or mixed, cultural groups that settled mostly in the Appalachians. Because there had been so much intermarriage early on, antebellum Southern lawmakers resisted a one-drop rule. However, during the Jim Crow era, there was a push for miscegenation laws that made defining races legally necessary. The Black Power movements of the 1960s also adopted the one-drop rule, counting anyone with black African ancestry as African-American.