A very interesting online resource to learn about the unsaid and hugely unacknowledged history of the United States.

https://sundown.tougaloo.edu/index.php

https://sundown.tougaloo.edu/content.php?file=sundowntowns-whitemap.html

A sundown town is not just a place where something racist happened. It is an entire community (or even county) that for decades was "all white" on purpose. "All white" is in quotes because some towns allowed one black family to remain when they drove out the rest. Also, institutionalized persons (in prisons, hospitals, colleges, etc.), live-in servants (in white households), and black or interracial children (in white households) do not violate the taboo.

"On purpose" does not require a formal ordinance. If, for example, a black family tried to move in, encountered considerable hostility, and left, that would qualify the town as "sundown." Note that some sundown towns kept out Chinese Americans, Jews, Mexican Americans, Native Americans, even Mormons.