 In such a case the contract is not varied, but merely applied.Oral evidence may be given, therefore,


but not, apparently, where the language is clear and explicit, or where the meaning of the word is a matter of law.Parol evidence is always admissible to show that the written contract is only part of the whole transaction, and that a separate oral agreement made at the same time was not incorporated in the written agreement—provided that the oral agreement referred to a matter on which the document is silent, and is not inconsistent with the terms of the written contract.