If the parties never intended the written contract to be their full understanding – if they intended it to be partly oral – the rule does not apply. If the document is fully integrated, no extrinsic evidence is permitted to change the terms of the agreement, even if the amendment is in addition to the existing conditions, and not a contradiction on their part. If the contract is partially integrated, additional consistent preconditions may be displayed. It is the duty of the party that wants to exclude parol evidence to show that the contract should be integrated. It is not always an easy task. In order to prevent a party from later introducing extrinsic evidence, in order to prove that there were previous agreements, the contract itself may recite that there was none. For example, here is the final clause of the National Basketball Association`s uniformity contract: „This agreement contains the full agreement between the parties and there are no written or oral incentives, promises or agreements, except like this one. Such a clause is called the merger clauseA clause in the contract that states that the written agreement contains the full understanding and intent of the parties. Unlike Minerva, who, in Greek mythology, is quite out of Zeus` forehead, the treaties do not appear on a mark-up that commemorates on paper.

Almost without exception, the negotiations precede the conclusion of an agreement. People write letters, talk by phone, meet face-to-face, send emails and exchange opinions on what they want and how they respond. They can even lie in a dutunal way, despise themselves and make promises that they know they cannot or will not keep so as not to kill contract negotiations. During these discussions, they will be able to conclude interim agreements, some of which will ultimately be reflected in the final treaty, some of which will be rejected along the way and some of which may not be included in the final agreement, but will not be refuted by it. Whether these earlier agreements should be taken into account is a recurring problem. Despite its apparent severity, the Parol rule of evidence does not negate all prior agreements or statements and does not exclude their use as evidence.