The court's primary duty when construing an unambiguous deed is to ascertain the intent of the parties from the "four corners" of the deed. Luckel, 819 S.W.2d at 461. Under the "four corners rule," the intention of the parties, especially that of the grantor, will be gathered from the instrument as a whole and not from isolated parts of the instrument. Plainsman Trading Co. v. Crews, 898 S.W.2d 786, 789 (Tex. 1995); Texas Pac. Coal & Oil Co. v. Masterson, 160 Tex. 548, 334 S.W.2d 436, 439 (Tex. 1960). The question of whether a contract is ambiguous is one of law for the court. R & P Enterprises v. LaGuarta, Gavrel & Kirk, Inc., 596 S.W.2d 517, 519 (Tex. 1980). In the interpretation of contracts, the primary concern of courts is to ascertain and to give effect to the intentions of the parties as expressed in the instrument. Id. If a written instrument is so worded that a court may properly give it a certain or definite legal meaning or interpretation, it is not ambiguous. Id. If, after applying the established rules of interpretation, a written instrument remains reasonably susceptible to more than one meaning, extraneous evidence is admissible to determine the true meaning of the instrument. Id. Where there is no ambiguity, disagreement over the interpretation of the reservation will not make it ambiguous, Sun Oil Co. v. Madeley, 626 S.W.2d 726, 727 (Tex. 1981), for a party's construction is immaterial. Id. at 732.