Wis. Stat. § 63.18
The mayor of each 1st class city, whether the city is incorporated by special act of the legislature or under the general laws of the state, shall, before June 15 in the year following the first state or national census, showing the city to belong to the 1st class, appoint 5 persons, citizens and residents of the city, who shall constitute and be known as the board of city service commissioners of the city, and shall designate one of the persons so appointed to serve for a term of 5 years, one for a term of 4 years, one for a term of 3 years, one for a term of 2 years, and one for a term of one year, from the first Monday of July in the year of their appointment and until their respective successors are appointed and qualified. In cities having a board of city service commissioners the members of such board shall hold office and continue to be a member of such commission until the expiration of the term or terms of such member or members, and in each and every year after such first appointment, the mayor shall, in like manner, in the month of June, appoint one person as the successor of the commissioner whose term shall expire in that year, to serve as such commissioner for 5 years from the first Monday of July then next ensuing, and until a successor is appointed and qualified. The commission shall, at a meeting in July of each year, elect one member to act as president and one member to act as vice president, each for a term of one year, and until a successor is duly elected. Three commissioners shall constitute a quorum necessary for the transaction of business. Any vacancy in the office of commissioner occurring during the term shall be filled for the unexpired term by appointment by the mayor and all appointments, both original and to fill vacancies, shall be so made that not more than 2 commissioners shall at the time of the appointment be members of the same political party. The commissioners shall hold no other lucrative office or employment under the United States, this state, or any municipal corporation or political division thereof, and each commissioner shall before entering upon the discharge of the duties of office and within 10 days after receiving notice of appointment, take and subscribe the oath of office prescribed by article IV, section 28, of the constitution, and file the oath, duly certified by the officer administering it with the clerk of his or her city.
Wis. Stat. § 63.18