10 ILCS 5/5-28
The original registration record cards shall remain permanently in the office of the county clerk or election authority except as destroyed as provided in Section 5-6; shall be filed alphabetically without regard to precincts; and shall be known as the master file. The master file may be kept in a computer-based voter registration file or paper format, provided a secondary digital back-up is kept off site. The digital file shall be searchable and remain current with all registration activity conducted by the county clerk or election authority. The duplicate registration record cards shall constitute the official registry of voters for all elections and shall be filed by precincts and townships. The duplicate cards for use in conducting elections shall be delivered to the judges of election by the county clerk in a suitable binder or other device, which shall be locked and sealed in accordance with the directions to be given by the county clerk and shall also be suitably indexed for convenient use by the precinct officers. The precinct files shall be delivered to the judges of election for use at the polls for elections at the same time as the official ballots are delivered to them, and shall be returned to the county clerk by the judges of election within the time provided for the return of the official ballots. The county clerk shall determine the manner of return and delivery of such file.
10 ILCS 5/5-28