35 ILCS 200/21-30
Except as provided in this Section, Section 9-260, and Section 21-40, in counties with 3,000,000 or more inhabitants, by January 31 annually, estimated tax bills setting out the first installment of property taxes for the preceding year, payable in that year, shall be prepared and mailed. The first installment of taxes on the estimated tax bills shall be computed at (i) 50% of the total of each tax bill for the preceding year for taxes payable on or before December 31, 2009, and (ii) 55% of the total of each tax bill for the preceding year beginning with the first installment payable in 2010 . If, prior to the preparation of the estimated tax bills, a certificate of error has been either approved by a court on or before November 30 of the preceding year or certified pursuant to Section 14-15 on or before November 30 of the preceding year, then the first installment of taxes on the estimated tax bills shall be computed at (i) 50% of the total taxes for the preceding year as corrected by the certificate of error for taxes payable on or before December 31, 2009, and (ii) 55% of the total taxes for the preceding year, as corrected by the certificate of error, beginning with the first installment payable in 2010 . By June 30 annually, actual tax bills shall be prepared and mailed. These bills shall set out total taxes due and the amount of estimated taxes billed in the first installment, and shall state the balance of taxes due for that year as represented by the sum derived from subtracting the amount of the first installment from the total taxes due for that year.
The county board may provide by ordinance, in counties with 3,000,000 or more inhabitants, for taxes to be paid in 4 installments. For the levy year for which the ordinance is first effective and each subsequent year, estimated tax bills setting out the first, second, and third installment of taxes for the preceding year, payable in that year, shall be prepared and mailed not later than the date specified by ordinance. Each installment on estimated tax bills shall be computed at 25% of the total of each tax bill for the preceding year. By the date specified in the ordinance, actual tax bills shall be prepared and mailed. These bills shall set out total taxes due and the amount of estimated taxes billed in the first, second, and third installments and shall state the balance of taxes due for that year as represented by the sum derived from subtracting the amount of the estimated installments from the total taxes due for that year.
The county board of any county with less than 3,000,000 inhabitants may, by ordinance or resolution, adopt an accelerated method of tax billing. The county board may subsequently rescind the ordinance or resolution and revert to the method otherwise provided for in this Code.
35 ILCS 200/21-30