10 ILCS 5/24A-9.1
Whenever an electronic scanning process is utilized to automatically examine and count the votes on ballot sheets, the provisions of this Section shall apply. A voter shall cast a proper vote on a ballot sheet by making a mark in the designated area for the casting of a vote for any party or candidate or for or against any proposition. For this purpose, a mark is an intentional darkening of the designated area on the ballot sheet, and shall not be an "X", a check mark, or any other recognizable letter of the alphabet, number, or other symbol which can be recognized as an identifying mark.
Whenever the ballot sheet includes designated areas on both sides, the election authority shall provide an envelope, sleeve or other device to each voter by means of which the voter can deliver the voted ballot sheet to the ballot box without the votes indicated on the ballot sheet being visible to other persons in the polling place.
10 ILCS 5/24A-9.1