California Code, Commercial Code - COM § 3115

(a) “Incomplete instrument” means a signed writing, whether or not issued by the signer, the contents of which show at the time of signing that it is incomplete but that the signer intended it to be completed by the addition of words or numbers.

(b) Subject to subdivision (c), if an incomplete instrument is an instrument under Section 3104, it may be enforced according to its terms if it is not completed, or according to its terms as augmented by completion. If an incomplete instrument is not an instrument under Section 3104, but, after completion, the requirements of Section 3104 are met, the instrument may be enforced according to its terms as augmented by completion.

(c) If words or numbers are added to an incomplete instrument without authority of the signer, there is an alteration of the incomplete instrument under Section 3407.

(d) The burden of establishing that words or numbers were added to an incomplete instrument without authority of the signer is on the person asserting the lack of authority.