store Systems, Inc., 813 P.2d 736 (Colo.1991).
We granted certiorari to decide whether Thompson’s employment contract was ambiguous as a matter of law, and to determine whether public employees, such as school superintendents, have an implied right to compensation for unused vacation time at the expiration of their employment contracts, absent an agreement to the contrary.
II.
We first address whether Thompson’s employment contract is ambiguous on the issue of compensation for unused vacation time. Reviewing courts are not bound by a trial court’s decision on the ambiguity of a contract, which is a question of law. Alley v. McMath, 140 Colo. 600, 346 P.2d 304 (1959). To ascertain whether certain provisions of a contract are ambiguous, “the language used therein must be examined and construed in harmony with the plain and generally accepted meaning of the words employed and by reference to all the parts and provisions of the agreement and the nature of the transaction which forms its subject matter.” Christmas v. Cooley, 158 Colo. 297, 301, 406 P.2d 333, 335 (1965). A document is ambiguous “when it is reasonably susceptible to more than one meaning.” Northern Ins. Co. of New York v. Ekstrom, 784 P.2d 320, 323 (Colo.1989). In deciding whether a contract is ambiguous, a court “may consider extrinsic evidence bearing upon the meaning of the written terms, such as evidence of local usage and of the circumstances surrounding the making of the contract. However, the court may not consider the parties’ own extrinsic expressions of intent.” KN Energy, Inc. v. Great Western Sugar Co., 698 P.2d 769, 777 (Colo.1985) (citations omitted), cert. denied, 472 U.S. 1022, 105 S.Ct. 3489, 87 L.Ed.2d 623 (1985); see also Pepcol Mfg. Co. v. Denver Union Corp., 687 P.2d 1310, 1314 n. 3 (Colo.1984) (citing 4 S. Williston, A Treatise on the Law of Contracts § 601, at 311 (W. Jaeger ed. 1961)) (“In determining whether a contract is ambiguous, the court may conditionally admit extrinsic evidence on this issue.”).
Once a contract is determined to be ambiguous, its interpretation becomes an issue of fact for the trial court to decide in the same manner as other disputed factual issues. Union Rural Elec. Ass’n v. Public Utils. Comm’n, 661 P.2d 247 (Colo.1983); Pepcol Mfg. Co., 687 P.2d 1310. Only after a contract is deemed ambiguous may the trial court use extrinsic evidence to assist it in ascertaining the intent of the parties. Schmelzer v. Condit, 69 Colo. 405, 195 P. 323 (1920); see also Union Rural Elec. Ass’n, 661 P.2d 247; Pepcol Mfg. Co., 687 P.2d 1310. One type of extrinsic evidence that a trial court may use to interpret the ambiguous terms of a contract is parol evidence. Parol evidence is admissible to explain or supplement the terms of an agreement, but not to vary or contradict them. Regan v. Customcraft Homes, Inc., 170 Colo. 562, 463 P.2d 463 (1970); Hott v. Tillotson-Lewis Constr. Co., 682 P.2d 1220 (Colo.App.1983). A court should use parol evidence only when the agreement is so ambiguous that the intent of the parties is not clear. McMath, 140 Colo. 600, 346 P.2d 304.
III.
The contract is silent on the specific question of whether Thompson is entitled to compensation for unused vacation at the expiration of his contract. Silence does not by itself necessarily create ambiguity as a matter of law. Silence does create ambiguity, however, when it involves a matter naturally within the scope of the contract. Consolidated Bearings Co. v. Ehret-Krohn Corp., 913 F.2d 1224, 1233 (7th Cir.1990). Compensation for unused accrued vacation on expiration of Thompson’s contract is a matter naturally within the scope of the contract.
The vacation-compensation component of the agreement between Thompson and the School Board is also ambiguous as a matter of law because it is subject to more than one reasonable interpretation. When read together and compared, different sections