Illinois constitution
Article VII
Section 3

(c) Members of the Cook County Board shall be elected
from two districts, Chicago and that part of Cook County
outside Chicago, unless (1) a different method of election is
approved by a majority of votes cast in each of the two
districts in a county-wide referendum or (2) the Cook County
Board by ordinance divides the county into single member
districts from which members of the County Board resident in
each district are elected. If a different method of election
is adopted pursuant to option (1) the method of election may
thereafter be altered only pursuant to option (2) or by
county-wide referendum. A different method of election may be
adopted pursuant to option (2) only once and the method of
election may thereafter be altered only by county-wide
referendum.