in the united states, social and legal conventions developed over time by whites that classified individuals of mixed ancestry into simplified racial categories (gossett 1997), but these were always porous. the decennial censuses conducted since 1790, after slavery was well established in the united states, included classification of persons by race: white, black, mulatto, and indian (nobles 2000). but, the inclusion of mulatto was an explicit acknowledgement of mixed race. in addition, before the civil war, virginia and some other states had legal definition of "whiteness" that provided for people being classified as white if no more than 1/8 black. (for example, if not born into slavery, thomas jefferson's children by his slave sally hemings would have been classified as legally white, as they were 7/8 white by ancestry. three of the four surviving children entered white society as adults, and their descendants have identified as white.) in the late 18th and 19th centuries, people of mixed race often migrated to frontiers where societies were more open, and they might be accepted as white if satisfying obligations of citizenship.