502us2$26Z 01-22-99 08:32:58 PAGES OPINPGT Cite as: 502 U. S. 367 (1992) 369 Syllabus tion should be granted only if the party satisfies the heavy burden of convincing the court that it agreed to the decree in good faith, made a reasonable effort to comply, and should be relieved of the undertaking under Rule 60(b). Accordingly, on remand the District Court should consider whether the upsurge in inmate population was foreseen by petitioners. Despite that court’s statement that it was, the decree itself and aspects of the record indicate that the increase may have been unanticipated. To relieve petitioners from the promise to provide single cells for pretrial detainees based on the increased jail population does not necessarily violate the decree’s basic purpose of providing a remedy for what had been found—based on a variety of factors, including double celling—to be unconstitutional conditions in the old jail. The rule cannot be that modifications of one of a decree’s terms defeats its purpose, since modification would then be all but impossible. Thus, the District Court erred in holding that, even under a standard more flexible than Swift’s, modification of the single cell requirement was necessarily forbidden. Pp. 383–387. (b) A decree must be modified if one or more of the obligations placed upon the parties later becomes impermissible under federal law, and may be modified when the statutory or decisional law has changed to make legal what the decree was designed to prevent. The Bell holding, which made clear that double celling is not in all cases unconstitutional, was not, in and of itself, a change in law requiring modification of the decree at issue. Since that holding did not cast doubt on the legality of single celling, the possibility that such a holding would be issued must be viewed as having been immaterial to petitioners when they signed the decree; i. e., they preferred even in the event of such a holding to agree to a decree which called for providing single cells in the new jail. To hold that a clarification in the law automatically opens the door for relitigation of the merits of every affected decree would undermine the finality of such agreements and could serve as a disincentive to settle institutional reform litigation. Nevertheless, a decision that merely clarifies the law could constitute a change supporting modification if the parties had based their agreement on a misunderstanding of the governing law. The decree at issue declares that it “sets forth a program which is both constitutionally adequate and constitutionally required” (emphasis added), and if petitioners can establish on remand that the parties believed that single celling was constitutionally mandated, this misunderstanding could form a basis for modification. Pp. 387–390. (c) Once a moving party has established a change in fact or in law warranting modification of a consent decree, the district court should determine whether a proposed modification is suitably tailored to the