California Code, Government Code - GOV § 56329

(a) Three appointed by the board of supervisors from their own membership. The board of supervisors shall appoint a fourth supervisor who is 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 a 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) Two representing the general public appointed by the other three members of the commission.Appointment of the public member and alternate public member shall be subject to the affirmative vote of at least one of the members selected by each of the other appointing authorities.