10 ILCS 5/8-6
In legislative or representative districts wholly contained within counties having 2,000,000 or more inhabitants each member of each legislative or representative committee shall in its organization and proceedings be entitled to one vote for each ballot voted in that portion of his township or ward in the legislative or representative district by the primary electors of his party at the last primary at which members of the General Assembly were nominated. If a portion of the legislative or representative district is within a county containing 2,000,000 or more inhabitants then each legislative or representative committee member shall be entitled to vote as follows: (a) in the portion of the district lying within a county of 2,000,000 or more inhabitants, each committeeperson shall be entitled to one vote for each ballot voted in that portion of his township or ward in the legislative or representative district by primary electors of his party at the last primary at which township or ward committeepersons were elected; (b) in the portion of the district lying outside a county of 2,000,000 or more inhabitants, each chair of a county central committee shall be entitled to one vote for each ballot voted in that portion of his county in the legislative or representative district by the primary electors of his party at the last primary at which members of the General Assembly were nominated. In the remainder of the State, each member shall be entitled to cast one vote for each ballot voted in that portion of his county in the legislative or representative district by the primary electors of his party at the last primary at which members of the General Assembly were nominated. However, in counties under 2,000,000 population, if the legislative or representative district comprises only one county, or part of a county, each legislative or representative committee member shall be entitled to cast one vote.
10 ILCS 5/8-6