10 ILCS 5/4-15
Within 5 days after a person registers or transfers his registration with the office of the election authority , such election authority shall send by mail, and by electronic mail if the registrant has provided the election authority with an e-mail address, a certificate to such person setting forth the elector's name and address as it appears upon the registration record card, and shall request him in case of any error to present the certificate on or before the 7th day next ensuing at the office of the election authority in order to secure correction of the error. The certificate shall contain on the outside a request for the postmaster to return it within 5 days if it cannot be delivered to the addressee at the address given thereon. Upon the return by the post office of a certificate which it has been unable to deliver at the given address because the addressee cannot be found there or because no such address exists, a notice shall be at once sent through the United States mail to such person at the address appearing upon his registration record card requiring him to appear before the election authority , within 5 days, to answer questions touching his right to register. If the person notified fails to appear at the election authority's office within 5 days as directed or if he appears and fails to prove his right to register, the election authority shall mark his registration card as incomplete and he shall not be permitted to vote until his registration is satisfactorily completed.
If an elector possesses such a certificate valid on its face, if his name does not expressly appear to have been erased or withdrawn from the precinct list as corrected and revised as provided by Section 4-11 of this Article, if he makes an affidavit and attaches such certificate thereto, and if such affidavit substantially in the form prescribed in Section 17-10 of this Act is sworn to before a judge of election on suitable forms provided by the election authority for that purpose, such elector shall be permitted to vote even though his duplicate registration card is not to be found in the precinct binder and even though his name is not to be found upon the printed or any other list.
10 ILCS 5/4-15