 It follows that the other party to the case may in turn, by oral evidence, prove facts that show the written document did not constitute a contract at all—for example, that it was forged.As regards the contents or terms of the written agreement, however, there is a very definite rule of law, known as the parol evidence rule, which places strict limits on the evidence that may be adduced in aid of interpretation.The rule dictates that, where the parties intended their agreement to be fully and finally embodied in writing, evidence to contradict or vary the terms of the writing, or to add to or subtract from them, is inadmissible.