35 ILCS 200/16-115
In counties with 3,000,000 or more inhabitants, complaints that any property is overassessed or underassessed or is exempt may be made by any taxpayer. Complaints that any property is overassessed or underassessed or is exempt may be made by a taxing district that has an interest in the assessment to a board of review. All complaints shall be in writing, identify and describe the particular property, otherwise comply with the rules in force, be either signed by the complaining party or his or her attorney or, if filed electronically, signed with the electronic signature of the complaining party or his or her attorney, and be filed with the board of appeals (until the first Monday in December 1998 and the board of review beginning the first Monday in December 1998 and thereafter) in at least duplicate. The board shall forward one copy of each complaint to the county assessor.
Complaints by taxpayers and taxing districts and certificates of correction by the county assessor as provided in this Code shall be filed with the board according to townships on or before the dates specified in the notices given in Section 16-110.
35 ILCS 200/16-115