If the parties never intended the written contract to be their full understanding—if they intended it to be partly oral—then the rule does not apply. If the document is fully integrated, no extrinsic evidence will be permitted to modify the terms of the agreement, even if the modification is in addition to the existing terms, rather than a contradiction of them. If the contract is partially integrated, prior consistent additional terms may be shown. It is the duty of the party who wants to exclude the parol evidence to show the contract was intended to be integrated. That is not always an easy task. To prevent a party later from introducing extrinsic evidence to show that there were prior agreements, the contract itself can recite that there were none. Here, for example, is the final clause in the National Basketball Association Uniform Player Contract: “This agreement contains the entire agreement between the parties and there are no oral or written inducements, promises or agreements except as contained herein.” Such a clause is known as a merger clause.
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