The "one-drop rule"–that a person with any amount of known black African ancestry (however small or invisible) is considered black–is a classification that was used in parts of the United States. It is a colloquial term for a set of laws passed by 18 U.S. states between 1910 and 1931, many as a consequence of "Plessy v. Ferguson", a Supreme Court decision that upheld the concept of racial segregation by accepting a "separate but equal" argument. The set of laws was finally declared unconstitutional in 1967, when the Supreme Court ruled on anti-miscegenation laws while hearing "Loving v. Virginia", which also found that Virginia's Racial Integrity Act of 1924 was unconstitutional. The one-drop rule attempted to create a bifurcated system of either black or white regardless of a person's physical appearance, but sometimes failed as people with African ancestry sometimes passed as "white", as noted above. This contrasts with the more flexible social structures present in Latin America (derived from the Spanish colonial era "casta" system) where there were less clear-cut divisions between various ethnicities.