Cook County, Illinois has a geographic area that is located in the northern part of Illinois.
Wikipedia’s Cook County, Illinois page states:
Cook County is a county in the U.S. state of Illinois, with its county seat in Chicago. It is the second most populous county in the US after Los Angeles County, California. The county has 5,231,351 residents, which is 40.6 percent of all Illinois residents. Cook County’s population is larger than that of 29 individual U.S. states and the combined populations of the seven smallest states.
Cook County also has a variety of major expressways, tollways and state routes. Major expressways include the Dan Ryan Expressway, the Kennedy Expressway, the Edens Expressway, and the Eisenhower Expressway.