55 ILCS 5/2-6001
There shall be elected every 4 years by the legal voters of Cook County 17 county commissioners who shall hold their office respectively for the term of 4 years and until their successors are elected and qualified. Elections shall be held at the time provided by the general election law. Their term of office shall commence on the first Monday of the month following the month of their election. Ten of the commissioners shall be elected from the city of Chicago by the legal voters of that city, and 7 of the commissioners shall be elected from the towns outside of the city by the legal voters of those towns. If a vacancy occurs in the office of a commissioner from the city of Chicago, the remaining commissioners from the city of Chicago shall appoint a suitable person from the city of Chicago to fill the vacancy. If a vacancy occurs in the office of a commissioner from the towns outside the city of Chicago, the remaining commissioners from those towns shall appoint a suitable person from those towns to fill the vacancy.
55 ILCS 5/2-6001