The US Census has been counting African Americans as a separate race since 1820. But the way it's categorized African Americans, and what it's called them, has changed constantly.

Especially notable is that before 1960, Americans didn't even have the option of picking their own race; it was the census taker's job to do it for them. Which means that in 1890, for example, census takers were tasked with figuring out whether multiracial families counted as "mulatto," "quadroon," or "octoroon."

It's another illustration of how our understanding of what race is, and who belongs to which race, keeps shifting over time — even though people of every era are convinced that the racial divisions of their era are just scientific fact.

Watch: Race isn't biologically real