55 ILCS 5/3-5027
In counties with a population of less than 1,000,000, the recorder, pursuant to ordinance or resolution of the county board, may establish a permanent real estate index number system which shall describe all real estate in the county by township, Section, block, and parcel or lot, the street or post office address, if any, and street code number, if any, of such real estate. The recorder having established such index number system shall be the sole authority in the county to designate and assign index numbers and may establish and maintain cross indexes of numbers assigned with the complete legal description of the real estate to which such numbers relate. Such index number system may be used by the supervisor of assessments or board of assessors, as the case may be, in counties of less than 1,000,000 population for the purposes of assessment and collection of taxes. If a real estate index number system is being maintained by an authority other than the recorder on September 8, 1980, the county board may approve the adoption, modification or expansion of such system by the recorder and the recorder shall thereafter designate and assign all index numbers under said system. All indexes established hereunder shall be open to public inspection and shall be made available to the public during regular business hours.
55 ILCS 5/3-5027