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When do I have to complete Form IL-W-4?
If you are an employee, you must complete Form IL-W-4, Employee's and other Payee's Illinois Withholding Allowance Certificate and Instructions, on or before the date you start work or if you are making a change to your withholding allowances, so that your employer can withhold the correct amount of Illinois Income Tax from your pay. The amount withheld from your pay depends in part on the number of allowances you claim on this form.
If you do not file a completed Form IL-W-4 with your employer, fail to sign the form or include necessary information, or alter the form, your employer will withhold Illinois Income Tax on the entire amount of your compensation, without allowing any allowances.