65 ILCS 5/4-5-3
Except where candidates for commissioner are required to run for a specific office, the council, at the first regular meeting after an election of a member or members, shall designate by a majority vote one commissioner to be commissioner of accounts and finances, one to be commissioner of public health and safety, one to be commissioner of streets and public improvements, who ex officio shall be commissioner of public works, and one to be commissioner of public property. This designation may be changed by the council whenever it appears that the public service would be benefited thereby. The mayor shall be commissioner of public affairs. The commissioner of each department shall be superintendent of that department.
If the municipality has voted, as provided in Section 4-3-19, to require candidates for commissioner to run for a specific office, each commissioner shall be superintendent of the department bearing the same designation as the office to which he was elected. The mayor shall be commissioner of public affairs. The Commissioner of streets and public improvements shall be, ex-officio, commissioner of public works.
65 ILCS 5/4-5-3