35 ILCS 200/6-40
In all counties which elect a board of review, except counties with a county assessor elected under Section 3-45 and except counties with a board of review elected under Section 5-5, members shall be elected from 3 districts which are substantially equal in number of inhabitants and, to the extent practicable, equal in geographic area. On or before January 1 of the first year following a decennial census in which board members will be elected, the supervisor of assessments shall prepare and submit to the county board a map of the districts, designating each district as 1, 2 or 3. The county board shall adopt the map or make changes as it deems necessary and adopt the revised map on or before January 31. If no map is adopted by January 31, the map initially submitted by the supervisor of assessments shall constitute the districts from which members of the board of review shall be elected. As each term of a member of the board of review expires, a new member shall be elected from a district, beginning with district 1 and proceeding through district 3.
35 ILCS 200/6-40