In Pacific Gas & E. Co. v. G. W. Thomas Drayage & Rigging Co., Inc. (1968) 69 Cal.2d 33, the California Supreme Court explained the test for admissibility of extrinsic evidence to explain the meaning of a writing: "The test of admissibility of extrinsic evidence to explain the meaning of a written instrument is not whether it appears to the court to be plain and unambiguous on its face, but whether the offered evidence is relevant to prove a meaning to which the language of the instrument is reasonably susceptible." (Id. at p. 37.) "Although extrinsic evidence is not admissible to add to, detract from, or vary the terms of a written contract, these terms must first be determined before it can be decided whether or not extrinsic evidence is being offered for a prohibited purpose. The fact that the terms of an instrument appear clear to a judge does not preclude the possibility that the parties chose the language of the instrument to express different terms." (Id. at p. 39.) "Accordingly, rational interpretation requires at least a preliminary consideration of all credible evidence offered to prove the intention of the parties. Such evidence includes testimony as to the 'circumstances surrounding the making of the agreement . . .' so that the court can 'place itself in the same situation in which the parties found themselves at the time of contracting.' If the court decides, after considering this evidence, that the language of a contract, in the light of all the circumstances, is 'fairly susceptible of either one of the two interpretations contended for . . . , extrinsic evidence relevant to prove either of such meanings is admissible.'" (Id. at pp. 39-40.)