In the 1930’s the Home Owner’s Loan Corporation of America (HOLC) created actuarial maps of the United states. These maps were color coded — Green, Blue, Yellow, and Red — to reflect the amount of “risk” associated with home loans in those areas. The colors corresponded to “Best” (green), “Still Desirable” (blue), “Definitely Declining” (yellow), and “Hazardous” (red). Being in a green area made you likely to secure a federally-insured home mortgage, something that was effectively unavailable to red areas. Red areas were often associated with black populations, and these maps are where the term “redlining” comes from.
I live on Sunnyside Avenue in Arlington, Massachusetts. The neighborhood was built as two subdivisions in 1948, with 42 duplexes (84 homes total). These were starter homes with 792 square feet of finished space plus a basement with a garage. I affectionately refer to them as excellent specimens of mid-century slap-up. They were constructed in the mid 20th century, and the builder just kind of slapped them up.