Ragnar Benson, Inc. v. Hempfield Township Munic. Authority - Superior Court - Jan. 30, 2007
http://www.courts.state.pa.us/OpPosting/Superior/out/s65001_06.pdf
If a written contract is unambiguous and purports to be the entire agreement of the parties, and there is no claim of fraud or mistake, then oral testimony and prior written agreements or other writings are not admissible to explain or vary the terms of the contract. This is what is referred to as the parol evidence rule. See, e.g., Yocca v. Pittsburgh Steelers Sports, Inc., 854 A.2d 425 (Pa. 2004) for a "comprehensive review" of the rule.
