The parol evidence rule is not a rule of evidence, even though it governs admissibility of testimony

NATURE OF PAROL EVIDENCE RULES, EFFECT, AND POSSIBLE EXCEPTIONS 

The parol evidence rule is a rule of substantive law, not a rule of evidence. See Hubacek v. Ennis State Bank, 159 Tex. 166, 169, 317 S.W.2d 30, 31 (1958). When parties reduce an agreement to writing, the law of parol evidence presumes, in the absence of fraud, accident, or mistake, that any prior or contemporaneous oral or written agreements merged into the final written agreement. See DeClaire v. G&B McIntosh Family Ltd. P'ship, 260 S.W.3d 34, 45 (Tex. App.-Houston [1st Dist.] 2008, no pet.). Any provisions not set out in the writing are presumed to have been abandoned before execution of the agreement or, alternatively, they are presumed to have never been made. Id.

PAROL EVIDENCE TO BE DISREGARDED WHEN RULE APPLIES 

Evidence that violates the parol evidence rule has no legal effect and "merely constitutes proof of facts that are immaterial and inoperative." See Piper, Stiles & Ladd v. Fid. & Deposit Co. of Md., 435 S.W.2d 934, 940 (Tex. Civ. App.-Houston [1st Dist.] 1968, writ ref'd n.r.e.). Such evidence cannot be considered by the court when it construes the contract, even if the evidence is admitted without objection. See Johnson v. Driver, 198 S.W.3d 359, 364 (Tex. App.-Tyler 2006, no pet.).

WHAT EXCEPTIONS ARE THERE AND WHEN CAN THEY BE INVOKED? 

There are exceptions, however. Parol evidence may be admitted to show (1) that the contract was induced by fraud, accident, or mistake; (2) that an agreement was to become effective only upon certain contingencies; or (3) in the case of ambiguity, that the parties' true intentions differ from those expressed in the agreement. See Gonzalez v. United Bhd. of Carpenters & Joiners of Am., Local 551, 93 S.W.3d 208, 211 (Tex. App.-Houston [14th Dist.] 2002, no pet.).

Parol evidence may also be admitted under an additional exception to show collateral, contemporaneous agreements that are consistent with the underlying written agreement. See DeClaire, 260 S.W.3d at 45. However, this exception does not permit parol evidence that varies or contradicts the express or implied terms of the written agreement. Id.

SOURCE: HOUSTON COURT OF APPEALS - No. 14-13-00730-CV - 11/6/2014

ADDITIONAL CASELAW EXCERPT ON PAROL EVIDENCE RULE FROM CITED CASE: 

The parol evidence rule is a rule of substantive law. Hubacek v. Ennis State Bank, 159 Tex. 166, 317 S.W.2d 30, 31 (1958); Gonzalez v. United Bd. of Carpenters & Joiners, 93 S.W.3d 208, 211 (Tex. App.-Houston [14th Dist.] 2002, no pet.); Piper, Stiles & Ladd v. Fid. & Deposit Co., 435 S.W.2d 934, 940 (Tex.Civ.App.-Houston [1st Dist.] 1968, writ ref'd n.r.e.).

When parties reduce an agreement to writing, the law of parol evidence presumes, in the absence of fraud, accident, or mistake, that any prior or contemporaneous oral or written agreements merged into the written agreement and, therefore, that any provisions not set out in the writing were either abandoned before execution of the agreement or, alternatively, were never made and are thus excluded from consideration in interpreting the written agreement. See Hubacek, 317 S.W.2d at 31; Smith v. Smith, 794 S.W.2d 823, 827 (Tex.App.-Dallas 1990, no writ); Muhm v. Davis, 580 S.W.2d 98, 101 (Tex. Civ.App.-Houston [1st Dist.] 1979, writ ref'd n.r.e.). The terms of a promissory note cannot be contradicted or varied by parol evidence of a manner of payment other than as expressed in the note. Dameris v. Homestead Bank, 495 S.W.2d 52, 54 (Tex.Civ.App.-Houston [1st Dist.] 1973, no writ).

We review parol evidence questions de novo, as questions of law. City of Pasadena v. Gennedy, 125 S.W.3d 687, 691 (Tex.App.-Houston [1st Dist.] 2003, pet. denied). Evidence that violates the parol evidence rule has no legal effect and "merely constitutes proof of facts that are immaterial and inoperative." Piper, Stiles & Ladd, 435 S.W.2d at 940. Because all prior negotiations and agreements are presumed merged into the final agreement, parol evidence is not admissible to vary, alter, or supplement the terms of an otherwise unambiguous contract except to show (1) that the contract was induced by fraud, accident, or mistake; (2) that an agreement was to become effective only upon certain contingencies; or (3) in the case of ambiguity, that the parties' true intentions differ from those expressed in the agreement. See Messer v. Johnson, 422 S.W.2d 908, 912 (Tex.1968); Gonzalez, 93 S.W.3d at 211; Litton v. Hanley, 823 S.W.2d 428, 430 (Tex.App.-Houston [1st Dist.] 1992, no writ).

Parol evidence may also be admissible, under an additional exception, to show collateral, contemporaneous agreements that are consistent with the underlying agreement to be construed. See Hubacek, 317 S.W.2d at 31; see also Transit Enter., Inc. v. Addicks Tire & Auto Supply, Inc., 725 S.W.2d 459, 461 (Tex.App.-Houston [1st Dist.] 1987, no writ) (applying exception for collateral, consistent, contemporaneous agreements); Sherrod v. Bailey, 580 S.W.2d 24, 29 (Tex.Civ.App.-Houston [1st Dist.] 1979, writ ref'd n.r.e.) (same). But this latter exception does not permit parol evidence that varies or contradicts either the express terms or the implied terms of the written agreement. Hubacek, 317 S.W.2d at 31; Loe v. Murphy, 46*46 611 S.W.2d 449, 451-52 (Tex.Civ. App.-Dallas 1980, writ ref'd n.r.e.); NHA, Inc. v. Jones, 500 S.W.2d 940, 944-45 (Tex. Civ.App.-Fort Worth 1973, writ ref'd n.r.e.) (both citing Hubacek).