In Beneficial Etc. Ins. Co. v. Kurt Hitke & Co. (1956) 46 Cal. 2d 517, an insurance company sued an agent for a declaration of contractual rights allowing it to recover paid commissions. The trial court ruled in favor of the insurance company, and the agent appealed. "Defendant contends that none of the commissions paid to it monthly under paragraph 14 are returnable to plaintiff and that if the contract is not unequivocally subject to that construction, evidence offered by it to explain the contract and indicating such construction should have been admitted. Plaintiff, of course, contends the trial court's construction was correct and there was no ambiguity in the contract. Defendant offered to prove: (1) That the parties construed the contract during the time it was being carried out as urged by defendant and to that end plaintiff's vice president would testify, as he had by deposition, that the sums paid monthly to defendant were fully earned and not returnable at any time and that plaintiff had never demanded a return of those sums although they showed in a deficit position; (2) that under trade custom and usage in the insurance business with respect to general agency contracts, in which both parties were engaged, earned commissions are never returnable. The trial court rejected the offer. While the offer of proof is general and somewhat vague it is sufficient. The trial court had previously indicated that it would receive no extrinsic evidence of any kind bearing upon the construction of the contract." (Beneficial, supra, 46 Cal. 2d at p. 522.) In reversing, the court stated: "It should be clear, therefore, that extrinsic evidence was admissible here as an aid in construing the contract. The meaning given by the parties to the contract since its existence and their performance thereunder, the preliminary negotiations and the question of whether custom and usage in the insurance business has given meaning to the terms of the contract are all relevant to the interpretation of it and should enable the trial court to ascertain the meaning of the contract. Whether all that defendant offered to prove will be relevant and admissible cannot, of course, be now determined. That will depend upon the nature and character of the proof sought to be made." (Id. at p. 527.)