Knox County, Indiana in the context of Daviess County, Indiana

There are 102 counties in Illinois. The most populous of these is Cook County, the second-most populous county in the United States and the home of Chicago, while the least populous is Hardin County. The largest by land area is McLean County, while the smallest is Putnam County. Illinois's FIPS state code is 17 and its postal abbreviation is IL.

What is now Illinois was claimed as part of Illinois County, Virginia, between 1778 and 1782. Modern-day county formation dates to 1790 when the area was part of the Northwest Territory; two counties — St. Clair and Knox — were created at that time. Knox would later become a county in Indiana and is unrelated to the current Knox County in Illinois, while St. Clair would become the oldest county in Illinois. Successive territorial governments created 15 counties by the time Illinois achieved statehood in 1818. The last county created in the state, Ford County, was created by the Illinois government in 1859.

Vincennes is a city in and the county seat of Knox County, Indiana, United States. It is located on the lower Wabash River in the southwestern part of the state, nearly halfway between Evansville and Terre Haute. It was founded in 1732 by French fur traders, including the namesake François-Marie Bissot, Sieur de Vincennes. It is the oldest continually inhabited European settlement in Indiana and was its longest serving territorial capital. It is one of the oldest settlements west of the Appalachians. The population was 16,759 at the 2020 census.

Mount Carmel is a city in and the county seat of Wabash County, Illinois, United States. At the time of the 2020 census, the population was 7,015, and it is the largest city in the county. The next largest town in Wabash County is Allendale, population 458.

Located at the confluence of the Wabash, Patoka, and White rivers, Mount Carmel borders both Gibson and Knox counties of Indiana. A small community known informally as East Mount Carmel sits near the mouth of the Patoka River on the opposite (Gibson County) side of the Wabash River from Mount Carmel. Mount Carmel is 5 miles (8.0 km) northeast of the Forest of the Wabash, a National Natural Landmark within Beall Woods State Park and about a mile north-northeast of one of its main employers, the Gibson Generating Station. Mount Carmel is also the home of Wabash Valley College, part of the Community College System of Eastern Illinois.