California Code, Government Code - GOV § 3583

(a) An arrangement pursuant to which an employee may decide whether or not to join the recognized or certified employee organization, but which requires the employer to deduct from the wages or salary of any employee who does join, and pay to the employee organization which is the exclusive representative of that employee, the standard initiation fee, periodic dues, and general assessments of the organization for the duration of the written memorandum of understanding.  This arrangement shall not deprive the employee of the right to resign from the employee organization within a period of 30 days prior to the expiration of a written memorandum of understanding.

(b) The arrangement described in Section 3583.5.