The one-drop rule is a historical colloquial term in the United States for the social classification as Negro of individuals with any African ancestry; meaning any person with "one drop of Negro blood" was considered black. The principle of "invisible blackness" was 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.

Although racial segregation was adopted legally by southern states of the former Confederacy in the late 19th century, legislators resisted defining race by law. In 1895 in South Carolina during discussion, George D. Tillman said,

"...It is a scientific fact that there is not one full-blooded Caucasian on the floor of this convention. Every member has in him a certain mixture of… colored blood...It would be a cruel injustice and the source of endless litigation, of scandal, horror, feud, and bloodshed to undertake to annul or forbid marriage for a remote, perhaps obsolete trace of Negro blood. The doors would be open to scandal, malice and greed..."

The one-drop rule was not adopted as law until the 20th century: first in Tennessee in 1910 and in Virginia under the Racial Integrity Act of 1924 (following the passage of similar laws in several other states).

Before and during slavery, interracial relationships formed. In the antebellum years, free people of mixed race (free people of color) were considered legally white if individuals had up to one-eighth or one-quarter African ancestry (depending on the state). Most mixed-race people were absorbed into the majority culture based simply on appearance, associations and carrying out community responsibilities. These and community acceptance were the more important factors if a person's racial status were questioned, not his or her documented ancestry. Because of the social mobility of antebellum society in frontier areas, many people did not have documentation about their ancestors.

Based on DNA and historical evidence, Thomas Jefferson is widely believed to have fathered four surviving mixed-race children with his slave Sally Hemings, who was three-quarters white by ancestry. Her children were born into slavery but, as they were seven-eighths European in ancestry, they were legally white under Virginia law of the time.[4] Jefferson allowed the two oldest to escape in 1822 (freeing them legally would have been a public action which he avoided); the two youngest he freed in his 1826 will. Three of the four entered white society as adults, and all their descendants identified as white.

In the United States, White Americans usually classified people of partial Native American descent as Native American, in a similar example of hypodescent. In the early years of these types of unions and marriages, the fathers were usually white and the mothers Native American. Many Native American tribes had matrilineal descent systems, so within those communities, the children were considered by tradition to belong to the mother's clan. It was tradition for the mother's eldest brother to play a major role in the life of all her children, more important than that of the father within the tribe, as the uncle looked out for the children within his clan, the father belonging to a different clan. Among such tribes, mixed-race children could be absorbed into the culture, as had been the case traditionally with captives of any race who were adopted.

Among other tribes who had a patrilineal system, such as the Omaha, the full-bloods considered a half-breed a "white man" if he had a white father, which was their application of hypodescent. Half-breeds could belong officially to the Omaha tribe only if they were formally adopted into it.

In the United States, the concept of the one-drop rule has been chiefly applied by White Americans to those of sub-Saharan black African ancestry in the 20th century, as they were trying to maintain white supremacy. The poet Langston Hughes wrote in his 1940 memoir:

You see, unfortunately, I am not black. There are lots of different kinds of blood in our family. But here in the United States, the word 'Negro' is used to mean anyone who has any Negro blood at all in his veins. In Africa, the word is more pure. It means all Negro, therefore black. I am brown.