Illinois's FIPS state code is 17 and its postal abbreviation is IL.

Modern-day county formation dates to 1790 when the area was part of the Northwest Territory; two counties—St.

Cook County, established in 1831 and named for the early Illinois Attorney General Daniel Pope Cook, contained the absolute majority of the state's population in the first half of the 20th century and retains more than 40% of it as of the 2020 census[update].

While it does have a Lincoln city, Illinois does not have a county named after its favorite son, Abraham Lincoln; it does, however, have a Douglas County (founded 1859) named after his political rival Stephen A. Douglas.

It also has Calhoun County (founded 1825), named after John C. Calhoun, outspoken for his pro-slavery and pro-southern views in the years preceding the American Civil War.