In the context of a court's use of extrinsic evidence in determining the intent of the parties in entering the contract, the four corners rule is not identical to the parol evidence rule. The parol evidence rule has been explained as follows: the parol evidence rule generally precludes evidence of understandings, not reflected in a writing, reached before or at the time of its execution which would vary or modify its terms. Under the parol evidence rule, extrinsic or parol evidence concerning a prior or contemporaneous agreement is not admissible to vary or contradict a fully integrated writing. A party may not introduce parol or extrinsic evidence to show additional consistent terms of a contract unless the writing is incomplete or ambiguous.
The parties entered into a written contract under which the company was to install a security system. Later, a fire occurred at the homeowner's home, originating in the bath house. The fire activated the burglar alarm, to which the police responded. Noticing the fire, the police summoned the fire department. The homeowner sued the company for breach of contract and negligence in the installation of a security system. The homeowner alleged that it was the intent of the parties that the new security system would protect the entire premises. The circuit court, however, granted the security company's motion for summary judgment. The homeowner's appealed the case to the Appellate Court of Illinois.
Should the security company be liable for breach of contract?
The appellate court found that the four corners of the letter indicated that the parties intended that the company provide the enumerated services for the stated price. It was a final and complete integration of the agreement, and extrinsic evidence was barred. There was no evidence that the company breached the contract. The negligence claim failed, because it was based on alleged duties that exceeded the scope of the contract.