50 ILCS 45/25

Taxpayers have the right to know how tax payments and remittances covered by this Act will be applied to the tax liability owed to units of local government. Each unit of local government must provide, by ordinance, for the order of application of tax payments to tax liability, penalty, and interest, provided that in no case may a payment be applied to penalties due before it is applied to tax or interest. In the event that a unit of local government does not provide for application of payments, any payment or remittance received for a tax period will be applied first to tax for the period, then to interest due for the period, and then to penalties due for the period.