California Code, Government Code - GOV § 56327

(a) Two appointed by the board of supervisors from their own membership.  The board of supervisors shall appoint a third supervisor who shall serve as an alternate member of the commission.  The alternate member may serve and vote in place of any supervisor on the commission who is absent or who disqualifies himself or herself from participating in a meeting of the commission.  If the office of the regular county member becomes vacant, the alternate member may serve and vote in place of the former regular county member until the appointment and qualification of a regular county member to fill the vacancy.

(b) One appointed by the city in the county having the largest population, who is a member of the legislative body of the city, appointed by the city council.  The city council shall also appoint an alternate member who is a member of the legislative body of the city.  The alternate member may serve and vote in place of the regular city member if the city member is absent or disqualifies himself or herself from participating in a meeting of the commission.  If the office of the regular city member becomes vacant, the alternate member may serve and vote in place of the former regular city member until the appointment and qualification of a regular city member to fill the vacancy.

(c) One appointed by the cities in the county, who is a mayor or council member appointed by the city selection committee.  The city selection committee shall also appoint one alternate member who shall also be a mayor or council member and shall be appointed and serve pursuant to Section 56335.  The city selection committee is encouraged to appoint members to fairly represent the diversity of the cities in the county, with respect to population and geography.

(d) One representing the general public, appointed by the other four, or, if the commission is enlarged pursuant to Section 56327.3, the other six, members of the commission.  This member shall not be a resident of a city which is already represented on the commission.  The commission may also appoint an alternate public member, who shall not be a resident of a city represented on the commission, and who may serve and vote in the place of the regular public member if the regular public member is absent or disqualifies himself or herself from participating in a meeting of the commission.  If the office of the regular public member becomes vacant, the alternate member may serve and vote in place of the former regular public member until the appointment and qualification of a regular public member to fill the vacancy.