The Rule for Interpretation of Contracts in California
The applicable general rule for interpretation of contracts generally is definitively stated in Winet v. Price (1992) 4 Cal.App.4th 1159:
"Parol evidence is properly admitted to construe a written instrument when its language is ambiguous. The test of whether parol evidence is admissible to construe an ambiguity is not whether the language appears to the court to be unambiguous, but whether the evidence presented is relevant to prove a meaning to which the language is 'reasonably susceptible.' The decision whether to admit parol evidence involves a two-step process.
First, the court provisionally receives (without actually admitting) all credible evidence concerning the parties' intentions to determine 'ambiguity,' i.e., whether the language is 'reasonably susceptible' to the interpretation urged by a party. If in light of the extrinsic evidence the court decides the language is 'reasonably susceptible' to the interpretation urged, the extrinsic evidence is then admitted to aid in the second step -- interpreting the contract. Different standards of appellate review may be applicable to each of these two steps, depending upon the context in which an issue arises. The trial court's ruling on the threshold determination of 'ambiguity' (i.e., whether the proffered evidence is relevant to prove a meaning to which the language is reasonably susceptible) is a question of law, not of fact. Thus the threshold determination of ambiguity is subject to independent review.
The second step -- the ultimate construction placed upon the ambiguous language -- may call for differing standards of review, depending upon the parol evidence used to construe the contract. When the competent parol evidence is in conflict, and thus requires resolution of credibility issues, any reasonable construction will be upheld as long as it is supported by substantial evidence. However, when no parol evidence is introduced (requiring construction of the instrument solely based on its own language) or when the competent parol evidence is not conflicting, construction of the instrument is a question of law, and the appellate court will independently construe the writing. " (Id. at pp. 1165-1166.)