Cal. Gov. Code § 24203

Within six months after the first general election succeeding the creation of the county, the supervisors of any county created after July 1, 1907, shall classify themselves by lot into two classes, as nearly equal in number as possible, and the term of office of the class having the greater number shall expire in two years from the commencement of the term, and the term of office of the class having the lesser number shall terminate in four years from the commencement of the term.