This overlay map is from 1873 (2 years after the Chicago Fire) and is centered around the northside Polish neighborhood. It is useful for comparing old street names with their current versions. Like other maps of this series, you can pan (move the map in any direction) and zoom in or out. The overlay map is intended to work only over a narrow zoom range. So if you zoom in or zoom out too much, the overlay map will not be visible. The slider in the upper right hand corner (to the left of the Map button) lets you change how light or dark the overlay map is over the imagery.