65 ILCS 5/10-1-1
The mayor of each municipality which adopts this Division 1 as hereinafter provided shall, not less than 40 nor more than 90 days after the taking effect of this Division 1 in such municipality, appoint 3 persons, who shall constitute and be known as the civil service commissioners of such municipality, one for 3 years, one for 2 years and one for one year from the time of appointment and until their respective successors are appointed and qualified. In every year thereafter the mayor shall, in like manner, appoint one person as the successor of the commissioner whose term shall expire in that year to serve as such commissioner for 3 years and until his successor is appointed and qualified. Two commissioners shall constitute a quorum. All appointments to the commission, both original and to fill vacancies, shall be so made that not more than 2 members shall, at the time of appointment, be members of the same political party. The commissioners shall hold no other lucrative office or employment under the United States, the State of Illinois, or any municipal corporation or political division thereof. No person shall be appointed a commissioner who has been convicted of a felony under the laws of this State or comparable laws of any other state or the United States. Each commissioner, before entering upon the duties of his office, shall take the oath prescribed by the constitution of this state.
However, in any municipality having the commission form of municipal government, the appointment of civil service commissioners shall be made by the corporate authorities, and the corporate authorities may, by ordinance, provide that 5 commissioners shall be so appointed, one for one year, 2 for 2 years and 2 for 3 years. The corporate authorities shall appoint, in a like manner, the successors of the commissioners whose terms expire in that year to serve as commissioners for 3 years and until their successors are appointed and qualified. Three members shall constitute a quorum, and no more than 3 of the commissioners shall be of the same political party. If such municipality has adopted this Division 1 prior to the effective date of this amendatory Act of 1965, and subsequently provides, by ordinance, for 5 commissioners, 2 additional commissioners shall be so appointed, one for 2 years and one for 3 years, and successors shall be appointed in a like manner as commissions established after such effective date.
65 ILCS 5/10-1-1