California Code, Labor Code - LAB § 2751

(a) Whenever an employer enters into a contract of employment with an employee for services to be rendered within this state and the contemplated method of payment of the employee involves commissions, the contract shall be in writing and shall set forth the method by which the commissions shall be computed and paid.

(b) The employer shall give a signed copy of the contract to every employee who is a party thereto and shall obtain a signed receipt for the contract from each employee. In the case of a contract that expires and where the parties nevertheless continue to work under the terms of the expired contract, the contract terms are presumed to remain in full force and effect until the contract is superseded or employment is terminated by either party.

(c) As used in this section, “commissions” has the meaning set forth in Section 204.1.For purposes of this section only, “commission” does not include any of the following: