10 ILCS 5/4-21
For use in connection with referenda and the nonpartisan and consolidated elections, each election authority shall maintain permanent records of the boundaries of all political subdivisions partially or wholly within its jurisdiction and any districts thereof, and shall maintain permanent records indicating by tax extension number code for each registered voter the political subdivisions and any districts thereof in which that voter resides. Such records may be kept on the registration record cards or on separate registration lists, or if a method other than record coding by tax extension numbers as adopted by an election authority, such method shall be, approved by the State Board of Elections. Each political subdivision must, no later than 5 days after any redistricting, annexation, disconnection or other boundary change is adopted, give notice of any such adoption and the effective date of such act to each election authority having election jurisdiction over any of its former or new territory.
Each election authority must make available to election judges for use on election day, records indicating by tax extension number code or other method approved by the State Board of Elections for each registered voter, the political subdivisions in which that voter resides. For the purposes of election day use by election judges, such records must be kept on the registration record cards or on separate registration lists.
10 ILCS 5/4-21