10 ILCS 5/4-20
The original registration cards shall remain permanently in the office of the county clerk or election authority except as destroyed as provided in Section 4-5.01; 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 cards shall constitute the official registry of voters for all elections subject to the provisions of this Article 4, shall be filed by precincts alphabetically or geographically so as to correspond with the arrangement of the list for such precincts respectively, compiled pursuant to Section 4-11 of this Article, and shall be known as the precinct file. 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 duplicate cards 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 delivery and return of such duplicate cards, and shall at all other times retain them at his office except for such use of them as may be made under this Article with respect to registration not at the office of the county clerk.
10 ILCS 5/4-20