Chicago Historic Resources Survey
Between 1983 and 1995 the City surveyed every Chicago building put up before 1940, color-coding each one for age, physical integrity and possible significance. Red and orange are the top two of seven ratings, and they still carry weight in preservation review. A building in the survey is not necessarily a landmark.
Survey by the Commission on Chicago Landmarks. Chapter digitized by Preservation Chicago from the copies in our library; field maps from the Internet Archive.