Appellant husband seeks review of the trial court’s order modifying the visitation provisions of a separation agreement that had been incorporated into the divorce decree. We reverse.
Appellee wife (now Mrs. Fitt) brought an action to enforce and modify the divorce decree entered sixteen months earlier, contending that the separation agreement was ambiguous. She urged that the agreement be reformed to reflect the actual intent of the parties to have the children spend fifty percent of their time with her. Alternatively, she argued that a change in circumstances and the best interests of the children required the court to modify the original agreement.
The trial court “adopt[ed] the legal position set forth by the plaintiff”1 and ordered that “the children . . . spend alternating months of the school year with each parent. . . ” It expressed no conclusions regarding the agreement’s ambiguity and made no findings of fact concerning changed circumstances.
The father argues on appeal that the original separation agreement was not ambiguous and therefore needed no reformation, that there was no significant change in circumstances requiring modification of the agreement, and that the court’s order effectively created a “split custody” arrangement which is disfavored in this jurisdiction. Utley v. Utley, D.C.App., 364 A.2d 1167, 1170 (1976).2 He also argues that the trial judge failed to make sufficient “written findings of fact and separate conclusions of law” as required under Super.Ct. Dom.Rel.R. 52(a).
Our agreement with appellant’s position that the trial court violated Rule 52(a) would normally necessitate a remand for further findings of fact. However, having searched but found in the extensive record no set of facts (i. e., no change in circumstances) that could support the court’s final order modifying the divorce decree, remand *1381would be futile.3 We thus reverse the trial court’s decision as a clear abuse of discretion and reinstate the decree.4
I
The record shows that the parties were married in 1962. A daughter was born in 1964, and a son in 1966. The parties separated in 1975, executed a separation agreement on January 31, 1976, and were divorced by a decree entered on May 16, 1977, that incorporated the earlier separation agreement. The agreement gave legal custody of the two children to the husband and set forth the wife’s visitation rights as follows:
3. Each party states that he or she wishes to have the custody of the minor children of the parties. In compromise of these conflicting desires, the parties agree that the Husband shall have custody of the minor children of the parties, with the right reserved to the Wife to have the children, or either of them, with her at reasonable times and places so long as the exercise of such visitation rights does not unduly interfere with prior plans of or for the children. Without in any way derogating the rights the duties of the Husband as legal custodian of both children, the principal place of abode of Susan E. Rice shall be considered to be with the Wife.
4. The parties recognize that it is not practicable to formulate specific provisions as to all of the time which the children will spend with the Wife pursuant to the foregoing paragraph. On approximately a monthly basis, the parties will review their commitments and obligations, and the commitments and obligations of the children, and will agree upon the visitation schedule for the children for the forthcoming period, keeping in mind the children’s need for continuity and other needs and interests. Without limiting the scope of the Wife’s rights under the foregoing paragraph, nor the obligation of the parties under the preceding sentence of this paragraph, the parties agree that the Wife shall have the children with her from Friday after school until Monday morning on alternate weekends, and for one-half of all school holiday periods and of the summer vacation. Without being legally bound, the Husband represents that it is his expectation that the children will spend time with the Wife in addition to the specific periods provided, including one or more overnight visits during the school week, subject always to the best interests and welfare of the children.
The parties concede, and the trial court found, that at the time of this action, “the children spen[t] every other weekend with [appellee], as well as one-half of the vacations.” Appellee introduced charts at trial, prepared from her daily calendars, showing that during the period between entry of the decree and her filing of this action her visitation time with the children changed only once, dropping from an average of thirty-eight percent before her remarriage to thirty-six percent afterwards. (This excludes the 1977 summer vacation period when her visitation time was set at fifty percent under the separation agreement.)
II
Our first inquiry is whether any allegedly ambiguous term or terms in the parties’ separation agreement required the trial court’s interpretation, or whether the entire agreement was so ambiguous that the agreed upon visitation arrangement required modification.
Appellee alleges that the term “reasonable times and places” in Section 3 of the agreement is inadequately defined, and that the phrase “without limiting the scope,” (referring to her additional visitation rights *1382beyond those specifically provided in Section 4) is also unclear. She apparently also alleges that appellant’s failure to agree with her upon visitation scheduling on “a monthly basis” constituted a breach of the agreement. She claims that the agreement’s ambiguity permits the introduction of extrinsic (parol) evidence to demonstrate her contention that the parties intended a 50/50 visitation arrangement.
Appellant argues that these are merely boiler-plate terms and that the agreement clearly sets forth the parties’ responsibilities and rights.
We find this separation agreement clear and understandable,5 despite the flexibility and discretion it grants to appellant. As we construe it, visitation above the specified minimum is left to the appellant’s discretion: he may refuse to allow any additional visitation that “unduly interferes” with the children’s plans or he may, alternatively, choose to allow additional visitation “[without [however] being legally bound” to do so. The constraint on his discretion is self-evident and inherent in his duties and responsibilities as custodian; thus his actions are “subject always to the best interests and welfare of the children.”
The rationale for such a flexible and discretionary visitation arrangement, which we understand is not an unusual one, is set forth in the agreement, which states that “the parties recognize that it is not practical to formulate specific provisions” regarding visitation.
Had the wife desired to convert the husband’s “expectation,” that visitation beyond the stated minimum would be granted, into a mandatory obligation, she had every opportunity to do so.6 She chose not to. Instead, she subscribed to the merger clause’s presumption that the “agreement contains the entire understanding of the parties” and agreed that the parties would “not be bound by any representations, warranties, promises, covenants or understandings other than those expressly set forth. . . ” Furthermore, she acknowledged, by signing the separation agreement, that she “fully [understood] everything set forth . that [she] sought and obtained independent legal advice . . . and [was] fully informed of all legal rights and liabilities . [and] believe[d] the agreement to be fair, just and reasonable. . . . ”
Because the written language of the separation agreement is “susceptible of a clear and definite understanding,” Minmar Builders, Inc. v. Beltway Excavators, Inc., D.C.App., 246 A.2d 784, 786 (1968), and neither fraud, duress, nor mistake is alleged, the trial court’s admission of parol evidence to vary the express terms of the integrated agreement was erroneous. “If this were permitted, [agreements] would not be worth the paper on which they are written.” Tonn v. Philco Corp., D.C.App., 241 A.2d 442, 445 (1968) (citations omitted). Absent a finding of fact that the agreement was ambiguous or executed through fraud, duress or mistake, the court’s modification of the agreement to conform to extrinsic evidence of the parties’ intent is an abuse of discretion. See Reynolds v. Reynolds, D.C.App., 415 A.2d 535 at 537 (1980).
III
Appellee alternatively argues that changed circumstances — apparently, appellant’s reduction of her visitation time— mandated judicial modification of the original divorce decree.
Appellant contends that there was no material change in circumstances other than appellee’s dissatisfaction with the agree*1383ment and her feeling that she is stigmatized by anything short of 50/50 visitation rights. He also notes that the children’s best interests were never harmed by the visitation agreement and that no expert testimony regarding their well-being was before the court.7
Although it is indisputable that the courts have continuing jurisdiction to modify custody arrangements in the best interests of the children, Alves v. Alves, D.C.App., 262 A.2d 111, 117-18, 51 A.L.R.3d 213 (1970) (citing Demonet v. Burkart, 23 App.D.C. 308 (1904)), it is equally beyond dispute that the modification must be based upon changed circumstances which occurred after the divorce decree was entered and were not contemplated by the parties at the time. McGehee v. Maxfield, D.C.App., 256 A.2d 576, 578-79 (1969); Dawn v. Dawn, 90 U.S.App.D.C. 226, 228, 194 F.2d 895, 897 (1952). See Clark, Law of Domestic Relations § 17.7, at 597 et seq. (1968).
Any change justifying amendment of the decree must be both substantial and material to the child’s welfare and best interests. Dawn v. Dawn, supra at 228, 194 F.2d at 897. Actions for modification must not be used as procedural devices for reviewing the equities of the prior decree. Hamilton v. Hamilton, D.C.App., 247 A.2d 421, 423 (1968) (citing 27B C.J.S. Divorce § 322(2), at 695 (1959)).
Thus, the burden of proof that the changes alleged are material is placed upon the party seeking modification. Tennyson v. Tennyson, D.C.App., 381 A.2d 264, 266 (1977) (citing Hamilton v. Hamilton, supra at 42). Clark, supra at 600, explains the rationale for this rule:
[C]ourts are aware of [the] danger [that children will be disturbed by changes in custody] and are generally reluctant to modify existing custody arrangements. . The only cure for [the] evil [of prolonged custody battles] is continued careful scrutiny of each case at the trial court level, coupled with the imposition of a heavy burden of persuasion upon the party asking for the modification.
Appellee did not meet her burden of proof. The only change since the entry of the decree that was alleged was a minimal reduction in visitation well within the range contemplated by the decree.
Furthermore, even had she, arguendo, sustained her burden, the particular modification of the decree chosen by the trial court would have to be set aside as inadequately supported by the findings (or the record).
Although the specific modification of a divorce decree chosen by the court is within the court’s discretion, it must be supported by the findings and rationally related to the changed circumstance it purports to counter. That is, the modification must be reasonably calculated to promote the child’s best interest and welfare, which is the “controlling consideration.” Utley v. Utley, supra at 1170. See Coles v. Coles, D.C.App., 204 A.2d 330, 331 (1964).
The trial court here made no findings of fact which indicate that this 50/50 visitation arrangement is justified, other than a general assertion, under the heading “Factual Background,” that “[b]oth parties agree that there exists a need for greater certainty as far as the ability to schedule the children’s (and their own) activities.”8
The court’s recognition of a need for certainty does not mandate that it order any *1384one outcome; it merely requires some resolution to the problem presented. In our estimation, sustaining the validity of the original agreement between the parties would accomplish the same purpose.
Since there is no indication in the record, and no finding by the trial court, that the children's interests were adversely affected, either by the original visitation agreement or by the alleged change in circumstances, amounting to a slight reduction in appel-lee’s visitation time, and, furthermore, there is no evidence to support the trial court’s conclusion that a 50/50 division of the children’s time was calculated to serve their best interests, appellee did not meet her burden of proof and the trial court abused its discretion in granting her relief.
IV
This court has held, in McGehee v. Maxfield, supra, that a change reasonably contemplated by the parties when the agreement was made (as was the appellant’s discretion to grant additional visitation here) does not warrant a modification of the agreement (there, an express transfer of custody), if the existing arrangement is not proven to be other than in the best interest of the children. Id. at 578-79.
Having concluded that the parties’ separation agreement was not ambiguous and that appellee failed to show a material subsequent change in circumstances justifying the modification of the decree ordered by the trial court, we reverse the court’s order and reinstate the separation agreement as incorporated into the parties’ divorce decree.
So ordered.
. The confusion in plaintiffs legal argument must, therefore, be ascribed to the trial court. Not only does it seem to assign error to the trial court that granted the original divorce decree, it erroneously cites 24 Am.Jur.2d Divorce and Separation § 803 (1966), as authority for the proposition that any discretion over visitation privileges invalidates the entire visitation arrangement. See Willey v. Willey, 253 Iowa 1294, 115 N.W.2d 833, 838 (1962). Furthermore, it misapplies McGehee v. Maxfield, D.C.App., 256 A.2d 576 (1969), which held only that (parol) evidence regarding the intent of the parties is admissible to explain what meaning both parties meant to give to an ambiguous contract term. Parol evidence is admissible only to explain ambiguous contract terms or to invalidate a contract on grounds of fraud, duress or mutual mistake. Minmar Builders, Inc. v. Beltway Excavators, Inc., D.C.App., 246 A.2d 784, 786 (1968) (citations omitted). The “terms of an agreement are not rendered ambiguous by the mere fact that the parties do not agree upon their proper construction.” Sundown, Inc. v. Canal Square Associates, D.C.App., 390 A.2d 421, 432 (1978) (citations omitted). See infra at II.
. Utley, in dictum, restated the familiar proposition that
Generally, divided custody of a child of tender years is not favored. Perhaps the reason for this is that an orderly, meaningful, and reasonably secure family life is crucial during the formative years of a child’s life. A happy and normal family life is often impossible of accomplishment when a child of tender years is subjected to the frustrating experience of divided custody especially when in the process he is shifted from home to home, from city to city, or from one family environment to another. [Utley v. Utley, supra at 1170 (citations omitted).]
We need not decide the applicability of Utley to this case, since our reversal of the trial court’s decision rests on other grounds. In any event, the father’s continuing “custody” of the children was never directly challenged by the court or by the parties.
. See Utley v. Utley, supra at 1170.
. The trial court’s modification order was stayed by this court pending appeal.
. We note that there is no reference anywhere in the agreement to a fifty percent allocation of visitation time with appellee.
. Appellee’s attorney was allowed to testify that he advised his client to sign the separation agreement in reliance on appellant’s alleged oral “assurance” that appellee would be allowed fifty percent visitation rights, despite the fact that the agreement itself did not reflect that supposed understanding.
. The children requested and were granted permission not to appear before or be interrogated by the court. As appellee’s brief states, “Neither party called expert witnesses to testify concerning the best interest of the children.”
. Later in the order, this “Factual Background” is referred to as the “Findings of Fact” and the court makes “Conclusions of Law” based on these “findings,” including the same assertion that “there exists a need for greater certainty regarding [their] schedules.” In addition to ordering an alternating month visitation schedule, the court orders “that neither parent shall do anything in any way which would hinder the *1384love of the children for the other parent.” There are absolutely no findings in “Factual Background” or elsewhere in the order even remotely relevant to this part of the order.