This essay introduces the three legal rules that emerged in the early 1800s for deciding if you were a member of the Black endogamous group or a member of the White endogamous group: physical appearance, blood fraction, and association. It comprises three topics: Slavery Depended on Matrilineal Descent, Not on the Color Line shows that endogamous group membership neither affected nor was affected by slave status. Slave status was decided by a different rule entirely: the rule of matrilineal descent. The Color Line Became Legally Important Around 1800 explains why it became increasingly necessary for courts to decide whether someone was White or Black. At first, it was only to decide where the burden of proof fell in slavery cases, but state legislatures soon passed dozens of laws requiring distinctions across the color line. Physical Appearance, Blood Fraction, Association presents with examples, the strengths and weaknesses of each of the three rules that were applied in court.