The Fox Valley Genealogical Society focuses on genealogical information in parts of four counties in Illinois. They are DuPage, Kane, Kendall, and Will Counties (map courtesy of zeemaps.com).
The primary townships we cover are Aurora (Kane Co.), Naperville (DuPage Co.), Lisle (DuPage Co.), Downers Grove (DuPage Co.), Oswego (Kendall Co.), Wheatland (Will Co.), DuPage (Will Co.), and Lemont (Will Co.). An old map of those townships:
The purple boundary cross in center of the county map above is at the intersection of Aurora, Naperville, Oswego, and Wheatland Townships on this map. Also Oswego Township now follows the south side of the Fox River about 2 miles further west.