Sundown Towns

Sundown towns are communities that for decades—formally or informally—kept out African Americans or other groups. They are so named because some marked their city limits with placards warning specific groups of people to stay away after the sun went down. This allowed maids and workmen to provide unskilled labor during the day. They came into existence in the late 19th century and were scattered throughout the nation, but more often were located in the northern states that were not pre-Civil War slave states. De facto sundown towns existed at least into the 1970s and there may still be towns today that try to keep people of color away.

Forgotten story of America's whites-only towns: an introduction to one man's search. One of James Loewen’s most significant achievements is to link physically violent forms of racism with legal and social stratagems white Americans have found to isolate themselves from people of color, indeed from people “different” in any way.
"I imagined I would find maybe ten of these communities in Illinois, where I planned more research than in any other single state, and perhaps fifty across the country. To my astonishment, I have found 500 sundown towns in Illinois alone—and now estimate that, by 1970, their peak, 10,000 existed in the United States." Author James Loewen. Was Your Town a Sundown Town?