California Code, Public Resources Code - PRC § 5515

Before calling the election the board of supervisors shall divide the proposed district into five or seven wards or subdistricts, the boundaries of which shall be so drawn that each contains approximately an equal number of electors.

The municipalities and any other territory included in the proposed district may be divided for the purpose of establishing the boundaries of the wards or subdistricts, if no election precinct is divided.