list of counties in Illinois

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Key facts

Location: State of Illinois
Populations: 3,588 ( Hardin ) – 5,194,625 ( Cook )
Areas: 172 square miles (450 km ) ( Putnam ) – 1,186 square miles (3,070 km ) ( McLean )
Government: County government
Subdivisions: 255 Precincts , 1,426 Townships

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Commons category: Counties of Illinois

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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.