the borders appear to have been set at th street and halsted street after the fire see this article i have to guess that city limits formed the other two boundaries but haven't found any information on that in the ten minutes i've been searching that same article indicates that the fire limits were expanded in after a second major fire to cover the whole city that may contradict the humboldt park article but i'm not sure in any case the fire limits survive on as chapter of the current chicago municipal code they cover all zoning districts within the corporate limits classified b through b and c through c as defined in the comprehensive zoning ordinance of may as amended together with an additional feet in all directions from the outer limits of each area or areas so classified this appears to mean that most business and commercial districts in chicago are governed by the fire limits regulations but it's hard to tell the zoning ordinance has been superseded and the city's online zoning map doesn't have the fire limits noted on it