The identification of Homer Plessy as black was largely based on the racial classification laws in Louisiana at the time. Plessy was of mixed race, but he identified as black according to the ‘one-drop rule,’ which stated that any person with even a small amount of African ancestry was considered black.
In 1892, Plessy deliberately sat in a ‘whites only’ railway car to challenge segregation laws. He was arrested when the conductor requested that he move to a different car. His appearance and self-identification under the prevailing social norms were enough for authorities to classify him as black, even though he was seven-eighths white.
This event led to the landmark Supreme Court case, Plessy v. Ferguson, which upheld racial segregation. The understanding of race and identity during this time was defined by societal norms and legal standards, which strictly categorized individuals based on their ancestry.