ELI5: Explain Like I'm 5

One-drop rule

The one-drop rule was an idea that said if a person had even one drop of black African blood in them, they would be considered "black" by the law in many places in the United States. This rule was used to decide how people were treated by society and even in court. It meant that if your ancestors were black, you were treated like a black person. This rule was wrong and hurtful to a lot of people.