Absent pleading and proof of ambiguity, fraud, accident or mistake, a written instrument presumes that all prior agreements of the parties relating to the transaction have been merged into the written instrument and, therefore, that it is fully integrated. Weinacht v. Phillips Coal Co. & Bledsoe, 673 S.W.2d 677, 679 (Tex. App.–Dallas 1984, no writ); Wilkins v. Bain, 615 S.W.2d 314, 314 (Tex. App.–Dallas 1981, no writ). When a writing is intended to be fully integrated or as a completed memorial of a legal transaction, the parol evidence rule excludes evidence of any prior or contemporaneous expressions of the parties relating to that transaction. Miller v. Kendall, 804 S.W.2d 933, 940 (Tex. App.–Houston [1st Dist.] 1990, no writ). The parol evidence rule is particularly applicable when the written instrument states that it is the “entire agreement between the parties,” or other similar language. Austin Shoe Stores v. The Elizabeth Co., 538 S.W.2d 677, 680 (Tex. App.–Waco 1976, writ ref’d n.r.e.). The parol evidence rule is not a rule of evidence, but rather a rule of substantive law. Testimony admitted in violation of this rule, even though not objected to, cannot be considered by the court. Peters v. Gifford-Hill & Co., 794 S.W.2d 856, 861 (Tex. App.–Dallas 1990, writ denied).

Integration and Parol Evidence