65 ILCS 5/4-5-2
The council and its members shall possess and exercise all executive, administrative, and legislative powers and duties now possessed and exercised by the executive, legislative, and administrative officers in municipalities which are treated as properly incorporated under this Code or which hereafter incorporate under this Code, except that in municipalities under the commission form of municipal government, the board of local improvements provided for by Article 9 shall remain a separate and distinct body, with all the rights, powers, and duties contained in Article 9.
The executive and administrative powers and duties in municipalities under the commission form of municipal government shall be distributed among 5 departments, as follows:
The council, by ordinance, (1) shall determine the powers of and duties to be performed by each department and shall assign them to the appropriate departments; (2) shall prescribe the powers and duties of officers and employees, and may assign officers and employees to one or more of the departments; (3) may require an officer or employee to perform duties in 2 or more departments; and (4) may make such rules and regulations as may be necessary or proper for the efficient and economical conduct of the business of the municipality.
65 ILCS 5/4-5-2