California Code, Government Code - GOV § 51305

If the contract results in a unification of a county department with a similar city department requiring a reduction of employees in either department and in a particular line of promotion, the reduction shall be made only from those employees most recently employed within the line of promotion without reference to any code number under which the employee is acting at the time of the reduction.  The rule of seniority shall be preserved without discrimination between employees of either local agency.