Allied Chemical & Alkali Workers of America, Local Union No. 1 v. Pittsburgh Plate Glass Co.

delivered the opinion of the Court.

Under the National Labor Relations Act, as amended, mandatory subjects of collective bargaining include pension and insurance benefits for active employees, 1 and an employer’s mid-term unilateral modification of such benefits constitutes an unfair labor practice. 2 This cause *160 presents the question whether a mid-term unilateral modification that concerns, not the benefits of active employees, but the benefits of already retired employees also constitutes an unfair labor practice. The National Labor Relations Board, one member dissenting, held that changes in retired employees' retirement benefits are embraced by the bargaining obligation and that an employer’s unilateral modification of them constitutes an unfair labor practice in violation of §§ 8 (a) (5) and (1) of the Act. 177 N. L. R. B. 911 (1969). 3 The Court of Appeals for the Sixth Circuit disagreed and refused to enforce the Board’s cease-and-desist order, 427 P. 2d 936 (1970). We granted certiorari, 401 U. S. 907 (1971). We affirm the judgment of the Court of Appeals.

I

Since 1949, Local 1, Allied Chemical and Alkali Workers of America, has been the exclusive bargaining representative for the employees “working” on hourly rates of pay at the Barberton, Ohio, facilities of respondent Pittsburgh Plate Glass Co. 4 In 1950, the Union and the Company negotiated an employee group health insurance plan, in which, it was orally agreed, retired employees could participate by contributing the required *161 premiums, to be deducted from their pension benefits. This program continued unchanged until 1962, except for an improvement unilaterally instituted by the Company in 1954 and another improvement negotiated in 1959.

In 1962 the Company agreed to contribute two dollars per month toward the cost of insurance premiums of employees who retired in the future and elected to participate in the medical plan. The parties also agreed at this time to make 65 the mandatory retirement age. In 1964 insurance benefits were again negotiated, and the Company agreed to increase its monthly contribution from two to four dollars, applicable to employees retiring after that date and also to pensioners who had retired since the effective date of the 1962 contract. It was agreed, however, that the Company might discontinue paying the two-dollar increase if Congress enacted a national health program.

In November 1965, Medicare, a national health program, was enacted, 79 Stat. 291, 42 U. S. C. § 1395 et seq. The 1964 contract was still in effect, and the Union sought mid-term bargaining to renegotiate insurance benefits for retired employees. The Company responded in March 1966 that, in its view, Medicare rendered the health insurance program useless because of a non-duplication-of-benefits provision in the Company’s insurance policy, and stated, without negotiating any change, that it was planning to (a) reclaim the additional two-dollar monthly contribution as of the effective date of Medicare; (b) cancel the program for retirees; and (c) substitute the payment of the three-dollar monthly subscription fee for supplemental Medicare coverage for each retired employee. 5

*162 The Union acknowledged that the Company had the contractual right to reduce its monthly contribution, but challenged its proposal unilaterally to substitute supplemental Medicare coverage for the negotiated health plan. The Company, as it had done during the 1959 negotiations without pressing the point, disputed the Union’s right to bargain in behalf of retired employees, but advised the Union that upon further consideration it had decided not to terminate the health plan for pensioners. The Company stated instead that it would write each retired employee, offering to pay the supplemental Medicare premium if the employee would withdraw from the negotiated plan. Despite the Union’s objections the Company did circulate its proposal to the retired employees, and 15 of 190 retirees elected to accept it. The Union thereupon filed unfair labor practice charges.

The Board held that although the Company was not required to engage in mid-term negotiations, the benefits of already retired employees could not be regarded as other than a mandatory subject of collective bargaining. The Board reasoned that “retired employees are 'employees’ within the meaning of the statute for the purposes of bargaining about changes in their retirement benefits . . . .” 177 N. L. R. B., at 912. Moreover, “retirement status is a substantial connection to the bargaining unit, for it is the culmination and the product of years of employment.” Id., at 914. Alternatively, the Board considered “bargaining about changes in retirement benefits for retired employees” as “within the contemplation of the statute because of the interest which active employees have in this subject . . . .” Id., at 912. Apparently in support of both theories, the Board noted that “[bargaining on benefits for workers already retired is an established aspect of current labor-management relations.” Id., at 916. The Board also held that the *163 Company's “establishment of a fixed, additional option in and of itself changed the negotiated plan of benefits” contrary to §§ 8 (d) and 8 (a)(5) of the Act. Id., at 918. Accordingly, the Company was ordered to cease and desist from refusing to bargain collectively about retirement benefits and from making unilateral adjustments in health insurance plans for retired employees without first negotiating in good faith with the Union. The Company was also required to rescind, at the Union’s request, any adjustment it had unilaterally instituted and to mail and post appropriate notices. 6

II

Section 1 of the National Labor Relations Act declares the policy of the United States to protect commerce “by encouraging the practice and procedure of collective bargaining and by protecting the exercise by workers of full freedom of association, self-organization, and designation of representatives of their own choosing, for the purpose of negotiating the terms and conditions of their employment. . . .” 49 Stat. 449, as amended, 29 U. S. C. § 151. To effectuate this policy, § 8 (a) (5) provides that it is an unfair labor practice for an employer “to refuse to bargain collectively with the representatives of his employees, subject to the provisions of section” 9 (a). 49 Stat. 453, as amended, 29 U. S. C. § 158 (a)(5). Section 8(d), in turn, defines “to bargain *164 collectively” as “the performance of the mutual obligation of the employer and the representative of the employees to meet at reasonable times and confer in good faith with respect to wages, hours, and other terms and conditions of employment...61 Stat. 142, 29 U. S. C. 1158(d). Finally, § 9 (a) declares: “Representatives designated or selected for the purposes of collective bargaining by the majority of the employees in a unit appropriate for such purposes, shall be the exclusive representatives of all the employees in such unit for the purposes of collective bargaining in respect to rates of pay, wages, hours of employment, or other conditions of employment . . . .” 49 Stat. 453, as amended, 29 U. S. C. § 159 (a).

Together, these provisions establish the obligation of the employer to bargain collectively, “with respect to wages, hours, and other terms and conditions of employment,” with “the representatives of his employees” designated or selected by the majority “in a unit appropriate for such purposes.” This obligation extends only to the “terms and conditions of employment” of the employer’s “employees” in the “unit appropriate for such purposes” that the union represents. See, e. g., Mine Workers v. Pennington, 381 U. S. 657, 666 (1965); NLRB v. Borg-Warner Corp., 356 U. S. 342 (1958); Packard Co. v. NLRB, 330 U. S. 485 (1947); Phelps Dodge Corp. v. NLRB, 313 U. S. 177, 192 (1941) (dictum); Pittsburgh Glass Co. v. NLRB, 313 U. S. 146 (1941). The Board found that benefits of already retired employees fell within these constraints on alternative theories. First, it held that pensioners are themselves “employees” and members of the bargaining unit, so that their benefits are a “term and condition” of their employment. 7

*165 The Court of Appeals, in contrast, held “that retirees are not ‘employees’ within the meaning of section 8 (a) (5) and . . . the Company was under no constraint to collectively bargain improvements in their benefits with the Union.” 427 F. 2d, at 942. The court reasoned, first, “[Retirement with this Company, as with most other companies, is a complete and final severance of employment. Upon retirement, employees are completely removed from the payroll and seniority lists, and thereafter they perform no services for the employer, are paid no wages, are under no restrictions as to other employment or activities, and have no rights or expectations of reemployment,” id., at 944; and, second, “[i]t has repeatedly been held that the scope of the bargaining unit controls the extent of the bargaining obligation .... [And] the unit certified by the Board as appropriate was composed . . . only of presumably active employees . . . .” Id., at 945. For the reasons that follow we agree with the Court of Appeals.

First. Section 2 (3) of the Act provides:

“The term ‘employee’ shall include any employee, and shall not be limited to the employees of a particular employer, unless this subchapter explicitly states otherwise, and shall include any individual whose work has ceased as a consequence of, or in connection with, any current labor dispute or because of any unfair labor practice, and who has not obtained any other regular and substantially equivalent employment . . . .” 49 Stat. 450, as amended, 29 U. S. C. § 152 (3).

*166 We have repeatedly affirmed that the task of determining the contours of the term “employee” “has been assigned primarily to the agency created by Congress to administer the Act.” NLRB v. Hearst Publications, 322 U. S. 111, 130 (1944). See also Iron Workers v. Perko, 373 U. S. 701, 706 (1963); NLRB v. Atkins & Co., 331 U. S. 398 (1947). But we have never immunized Board judgments from judicial review in this respect. “[T]he Board’s determination that specified persons are ‘employees’ under this Act is to be accepted if it has ‘warrant in the record’ and a reasonable basis in law.” NLRB v. Hearst Publications, supra, at 131.

In this cause we hold that the Board’s decision is not supported by the law. The Act, after all, as § 1 makes clear, is concerned with the disruption to commerce that arises from interference with the organization and collective-bargaining rights of “workers” — not those who have retired from the work force. The inequality of bargaining power that Congress sought to remedy was that of the “working” man, and the labor disputes that it ordered to be subjected to collective bargaining were those of employers and their active employees. Nowhere in the history of the National Labor Relations Act is there any evidence that retired workers are to be considered as within the ambit of the collective-bargaining obligations of the statute.

To the contrary, the legislative history of § 2 (3) itself indicates that the term “employee” is not to be stretched beyond its plain meaning embracing only those who work for another for hire. In NLRB v. Hearst Publications, supra, we sustained the Board’s finding that newsboys were “employees” rather than independent contractors. We said that “the broad language of the Act’s definitions, which in terms reject conventional limitations on such conceptions as ‘employee,’ . . . leaves no doubt that its applicability is to be determined broadly, in *167 doubtful situations, by underlying economic facts rather than technically and exclusively by previously established legal classifications.” The term “employee” “must be understood with reference to the purpose of the Act and the facts involved in the economic relationship.” 322 U. S., at 129. Congress reacted by specifically excluding from the definition of “employee” “any individual having the status of an independent contractor.” The House, which proposed the amendment, explained:

“An ‘employee,’ according to all standard dictionaries, according to the law as the courts have stated it, and according to the understanding of almost everyone, . . . means someone who works for another for hire. But in the case of National Labor Relations Board v. Hearst Publications, Inc. . . . , the Board . . . held independent merchants who bought newspapers from the publisher and hired people to sell them to be 'employees.' The people the merchants hired to sell the papers were ‘employees’ of the merchants, but holding the merchants to be ‘employees’ of the publisher of the papers was most far reaching. It must be presumed that when Congress passed the Labor Act, it intended words it used to have the meanings that they had when Congress passed the act, not new meanings that, 9 years later, the Labor Board might think up. In the law, there always has been a difference, and a big difference, between ‘employees’ and ‘independent contractors.’ ‘Employees’ work for wages or salaries under direct supervision. ... It is inconceivable that Congress, when it passed the act, authorized the Board to give to every word in the act whatever meaning it wished. On the contrary, Congress intended then, and it intends now, that the Board give to words not far-fetched meanings but ordinary *168 meanings.” H. R. Rep. No. 245, 80th Cong., 1st Sess., 18 (1947) (emphasis added).

See also 93 Cong. Rec. 6441-6442; H. R. Conf. Rep. No. 510, 80th Cong., 1st Sess., 32-33 (1947). The 1947 Taft-Hartley revision made clear that general agency principles could not be ignored in distinguishing “employees” from independent contractors. NLRB v. United Insurance Co., 390 U. S. 254, 256 (1968). Although Hearst Publications was thus repudiated, we do not think its approach has been totally discredited. In doubtful cases resort must still be had to economic and policy considerations to infuse § 2 (3) with meaning. But, as the House comments quoted above demonstrate, this is not a doubtful case. The ordinary meaning of “employee” does not include retired workers; retired employees have ceased to work for another for hire.

The decisions on which the Board relied in construing § 2 (3) to the contrary are wide of the mark. The Board enumerated “unfair labor practice situations where the statute has been applied to persons who have not been initially hired by an employer or whose employment has terminated. Illustrative are cases in which the Board has held that applicants for employment and registrants at hiring halls — who have never been hired in the first place — as well as persons who have quit or whose employers have gone out of business are 'employees’ embraced by the policies of the Act.” 177 N. L. R. B., at 913 (citations omitted). Yet all of these cases involved people who, unlike the pensioners here, were members of the active work force available for hire and at least in that sense could be identified as “employees.” No decision under the Act is cited, and none to our knowledge exists, in which an individual who has ceased work without expectation of further employment has been held to be an “employee.”

*169 The Board also found support for its position in decisions arising under § 302 (c) (5) of the Labor Management Relations Act, 61 Stat. 157, 29 U. S. C. § 186 (c) (5). Section 302 prohibits, inter alia, any payment by an employer to any representative of any of his employees. Subsection (c) (5) provides an exemption for payments to an employee trust fund established "for the sole and exclusive benefit of the employees of such employer" and administered by equal numbers of representatives of the employer and employees. The word "employee," as used in that provision, has been construed to include "current employees and persons who were . . . current employees but are now retired." Blassie v. Kroger Co., 345 F. 2d 58, 70 (CA8 1965). 8 The Board considered that it would be anomalous to hold "that retired employees are not `employees' whose ongoing benefits are fit subjects of bargaining under Section 8 (a) (5), while under [§ 302 (c)] they are `employees' for the purpose of administering the same health insurance benefits. It would create the further anomaly that a union would not be entitled to act as the representative of retired employees under Section 8 (a) (5), while subject to an explicit statutory duty to act as their representative under [§ 302 (c)]." 177 N. L. R. B., at 915. 9

Yet the rationale of Blassie is not at all in point. The question there was simply whether under § 302 (c) (5) retirees remain eligible for benefits of trust funds estab *170 lished during their active employment. The conclusion that they do was compelled by the fact that the contrary reading of the statute would have made illegal contributions to pension plans, which the statute expressly contemplates in subsections (A) and (C). 10 No comparable situation exists in this case. Furthermore, there is no anomaly in the conclusion that retired workers are “employees” within § 302 (c) (5) entitled to the benefits negotiated while they were active employees, but are not “employees” whose ongoing benefits are embraced by the bargaining obligation of § 8 (a)(5). Contrary to the Board’s assertion, the union’s role in the administration of the fund is of a far different order from its duties as collective-bargaining agent. To accept the Board’s reasoning that the union’s § 302 (c) (5) responsibilities dictate the scope of the § 8 (a) (5) collective- *171 bargaining obligation would be to allow the tail to wag the dog. 11

Second. Section 9 (a) of the Labor Relations Act accords representative status only to the labor organization selected or designated by the majority of employees in a "unit appropriate" "for the purposes of collective bargaining." Section 9 (b) goes on to direct the Labor Board to "decide in each case whether, in order to assure to employees the fullest freedom in exercising the rights guaranteed by this subchapter, the unit appropriate for the purposes of collective bargaining shall be the employer unit, craft unit, plant unit, or subdivision thereof . . . ." 49 Stat. 453, as amended, 29 U. S. C. § 159 (b). We have always recognized that, in making these determinations, the Board is accorded broad discretion. See NLRB v. Hearst Publications, 322 U. S., at 132-135; Pittsburgh Glass Co. v. NLRB, 313 U. S. 146 (1941). Moreover, the Board's findings of fact, if supported by substantial evidence, are conclusive. National Labor Relations Act, § 10 (e), 49 Stat. 454, as amended, 29 U. S. C. § 160 (e). But the Board's powers in respect of unit determinations are not without limits, and if its *172 decision "oversteps the law," Packard Co. v. NLRB, 330 U. S., at 491, it must be reversed.

In this cause, in addition to holding that pensioners are not “employees” within the meaning of the collective-bargaining obligations of the Act, we hold that they were not and could not be “employees” included in the bargaining unit. The unit determined by the Board to be appropriate was composed of “employees of the Employer’s plant . . . working on hourly rates, including group leaders who work on hourly rates of pay . . . Apart from whether retirees could be considered “employees” within this language, they obviously were not employees “working” or “who work” on hourly rates of pay. Although those terms may include persons on temporary or limited absence from work, such as employees on military duty, it would utterly destroy the function of language to read them as embracing those whose work has ceased with no expectation of return.

In any event, retirees could not properly be joined with the active employees in the unit that the Union represents. “As a standard, the Board must comply . . . with the requirement that the unit selected must be one to effectuate the policy of the act, the policy of efficient collective bargaining.” Pittsburgh Glass Co. v. NLRB, supra, at 165. The Board must also exercise care that the rights of employees under § 7 of the Act “to self-organization . . . [and] to bargain collectively through representatives of their own choosing” are duly respected. In line with these standards, the Board regards as its primary concern in resolving unit issues “to group together only employees who have substantial mutual interests in wages, hours, and other conditions of employment.” 15 NLRB Ann. Rep. 39 (1950). Such a mutuality of interest serves to assure the coherence among employees necessary for efficient collective bargaining and at the same time to prevent a function *173 ally distinct minority group of employees from being submerged in an overly large unit. See Kalamazoo Paper Box Corp., 136 N. L. R. B. 134, 137 (1962).

Here, even if, as the Board found, active and retired employees have a common concern in assuring that the latter’s benefits remain adequate, they plainly do not share a community of interests broad enough to justify inclusion of the retirees in the bargaining unit. Pensioners’ interests extend only to retirement benefits, to the exclusion of wage rates, hours, working conditions, and all other terms of active employment. Incorporation of such a limited-purpose constituency in the bargaining unit would create the potential for severe internal conflicts that would impair the unit’s ability to function and would disrupt the processes of collective bargaining. Moreover, the risk cannot be overlooked that union representatives on occasion might see fit to bargain for improved wages or other conditions favoring active employees at the expense of retirees’ benefits. 12

But we need not rely on our own assessment of the probable consequences of including retirees in the bargaining unit to conclude that the resulting unit would *174 be inappropriate. The Board itself has previously recognized that retirees do not have a sufficient interest to warrant participation in the election of a collective-bargaining agent. In Public Service Corp. of New Jersey, 72 N. L. R. B. 224, 229-230 (1947), for example, the Board stated:

“We have considerable doubt as to whether or not pensioners are employees within the meaning of Section 2 (3) of the Act, since they no longer perform any work for the Employers, and have little expectancy of resuming their former employment. In any event, even if pensioners were to be considered as employees, we believe that they lack a substantial community of interest with the employees who are presently in the active service of the Employers. Accordingly, we find that pensioners are ineligible to vote in the election.” 13

The Board argues, however, that the pensioners’ ineligibility to vote is not dispositive of their right to membership in the bargaining unit, since the franchise and the right to membership depend upon different levels of interest in the unit. 14 Yet in W. D. Byron & Sons of Maryland, Inc., 55 N. L. R. B. 172, 174-175 (1944), which the Board found controlling in Public Service Corp. of New Jersey, see 72 N. L. R. B., at 230 n. 10, the Board not merely held ineligible to vote, but expressly *175 excluded from the bargaining unit pensioners who had little expectation of further employment. In any event, it would be clearly inconsistent with the majority rule principle of the Act to deny a member of the unit at the time of an election a voice in the selection of his bargaining representative. 15 The Board’s own holdings thus compel the conclusion that a unit composed of active and retired workers would be inappropriate.

Third. The Board found that bargaining over pensioners’ rights has become an established industrial prac *176 tice. But industrial practice cannot alter the conclusions that retirees are neither “employees” nor bargaining unit members. The parties dispute whether a practice of bargaining over pensioners’ benefits exists and, if so, whether it reflects the views of labor and management that the subject is not merely a convenient but a mandatory topic of negotiation. 16 But even if industry commonly regards retirees’ benefits as a statutory subject of bargaining, that would at most, as we suggested in Fibreboard Corp. v. NLRB, 379 U. S. 203, 211 (1964), reflect the interests of employers and employees in the subject matter as well as its amenability to the collective-bargaining process; it would not be determinative. Common practice cannot change the law and make into bargaining unit “employees” those who are not.

III

Even if pensioners are not bargaining unit “employees,” are their benefits, nonetheless, a mandatory subject of collective bargaining as “terms and conditions of employment” of the active employees who remain in the unit? The Board held, alternatively, that they are, on the ground that they “vitally” affect the “terms and conditions of employment” of active employees principally by influencing the value of both their current and future benefits. 177 N. L. R. B., at 915. 17 The Board ex *177 plained: “It is not uncommon to group active and retired employees under a single health insurance contract with the result that ... it is the size and experience of the entire group which may determine insurance rates.” Ibid. Consequently, active employees may “benefit from the membership of retired employees in the group whose participation enlarges its size and might thereby lower costs per participant.” Ibid. Furthermore, the actual value of future benefits depends upon contingencies, such as inflation and changes in public law, which the parties cannot adequately anticipate and over which they have little or no control. By establishing a practice of repre *178 senting retired employees in resolving those contingencies as they arise, active workers can insure that their own retirement benefits will survive the passage of time. This, in turn, the Board contends, facilitates the peaceful settlement of disputes over active employees’ pension plans. The Board’s arguments are not insubstantial, but they do not withstand careful scrutiny.

Section 8 (d) of the Act, of course, does not immutably fix a list of subjects for mandatory bargaining. See, e. g., Fibreboard Corp. v. NLRB, supra, at 220-221 (STEWART, J., concurring); Richfield Oil Corp. v. NLRB, 97 U. S. App. D. C. 383, 389-390, 231 F. 2d 717, 723-724 (1956). But it does establish a limitation against which proposed topics must be measured. In general terms, the limitation includes only issues that settle an aspect of the relationship between the employer and employees. See, e. g., NLRB v. Borg-Warner Corp., 356 U. S. 342 (1958). Although normally matters involving individuals outside the employment relationship do not fall within that category, they are not wholly excluded. In Teamsters Union v. Oliver, 358 U. S. 283 (1959), for example, an agreement had been negotiated in the trucking industry, establishing a minimum rental that carriers would pay to truck owners who drove their own vehicles in the carriers' service in place of the latter's employees. Without determining whether the owner-drivers were themselves "employees," we held that the minimum rental was a mandatory subject of bargaining, and hence immune from state antitrust laws, because the term "was integral to the establishment of a stable wage structure for clearly covered employee-drivers." United States v. Drum, 368 U. S. 370, 382-383, n. 26 (1962). 18 Similarly, *179 in Fibreboard Corp. v. NLRB, supra, at 215, we held that "the type of `contracting out' involved in this case—the replacement of employees in the existing bargaining unit with those of an independent contractor to do the same work under similar conditions of employment—is a statutory subject of collective bargaining . . . ." As we said there, id., at 213, "the work of the employees in the bargaining unit was let out piecemeal in Oliver, whereas here the work of the entire unit has been contracted out.”

The Board urges that Oliver and Fibreboard provide the principle governing this cause. The Company, on the other hand, would distinguish those decisions on the ground that the unions there sought to protect employees from outside threats, not to represent the interests of third parties. We agree with the Board that the principle of Oliver and Fibreboard is relevant here; in each case the question is not whether the third-party concern is antagonistic to or compatible with the interests of bargaining-unit employees, but whether it vitally affects the “terms and conditions” of their employment. 19 But we disagree with the Board’s assessment of the significance of a change in retirees’ benefits to the “terms and conditions of employment” of active employees.

*180 The benefits that active workers may reap by including retired employees under the same health insurance contract are speculative and insubstantial at best. As the Board itself acknowledges in its brief, the relationship between the inclusion of retirees and the overall insurance rate is uncertain. Adding individuals increases the group experience and thereby generally tends to lower the rate, but including pensioners, who are likely to have higher medical expenses, may more than offset that effect. In any event, the impact one way or the other on the “terms and conditions of employment” of active employees is hardly comparable to the loss of jobs threatened in Oliver and Fibreboard. In Fibreboard, after holding that “the replacement of employees in the existing bargaining unit with those of an independent contractor to do the same work under similar conditions of employment” is a mandatory subject of bargaining, we noted that our decision did “not encompass other forms of 'contracting out' or ‘subcontracting’ which arise daily in our complex economy.” 379 U. S., at 215. The inclusion of retirees in the same insurance contract surely has even less impact on the “terms and conditions of employment” of active employees than some of the contracting activities that we excepted from our holding in Fibreboard.

The mitigation of future uncertainty and the facilitation of agreement on active employees’ retirement plans, that the Board said would follow from the union’s representation of pensioners, are equally problematical. To be sure, the future retirement benefits of active workers are part and parcel of their overall compensation and hence a well-established statutory subject of bargaining. Moreover, provisions of those plans to guard against future contingencies are equally subsumed under the collective-bargaining obligation. Under the Board’s *181 theory, active employees undertake to represent pensioners in order to protect their own retirement benefits, just as if they were bargaining for, say, a cost-of-living escalation clause. But there is a crucial difference. Having once found it advantageous to bargain for improvements in pensioners’ benefits, active workers are not forever thereafter bound to that view or obliged to negotiate in behalf of retirees again. 20 To the contrary, they are free to decide, for example, that current income is preferable to greater certainty in their own retirement benefits or, indeed, to their retirement benefits altogether. By advancing pensioners’ interests now, active employees, therefore, have no assurance that they will be the beneficiaries of similar representation when they retire. The insurance against future contingencies *182 that they may buy in negotiating benefits for retirees is thus a hazardous and, therefore, improbable investment, far different from a cost-of-living escalation clause that they could contractually enforce in court. See n. 20, supra,. We find, accordingly, that the effect that the Board asserts bargaining in behalf of pensioners would have on the negotiation of active employees’ retirement plans is too speculative a foundation on which to base an obligation to bargain.

Nor does the Board’s citation of industrial practice provide any ground for concluding otherwise. The Board states in its brief that “[n] either the bargaining representative nor the active employees . . . can help but recognize that the active employees of today are the retirees of tomorrow — indeed, such a realization undoubtedly underlies the widespread industrial practice of bargaining about benefits of those who have already retired . . . and explains the vigorous interest which the Union has taken in this case.” But accepting the Board’s finding that the industrial practice exists, we find nowhere a particle of evidence cited showing that the explanation for this lies in the concern of active workers for their own future retirement benefits.

We recognize that “classification of bargaining subjects as ‘terms [and] conditions of employment’ is a matter concerning which the Board has special expertise.” Meat Cutters v. Jewel Tea, 381 U. S. 676, 685-686 (1965). The Board’s holding in this cause, however, depends on the application of law to facts, and the legal standard to be applied is ultimately for the courts to decide and enforce. We think that in holding the “terms and conditions of employment” of active employees to be vitally affected by pensioners’ benefits, the Board here simply neglected to give the adverb its ordinary meaning. Cf. NLRB v. Brown, 380 U. S. 278, 292 (1965).

*183 IV

The question remains whether the Company committed an unfair labor practice by offering retirees an exchange for their withdrawal from the already negotiated health insurance plan. After defining “to bargain collectively” as meeting and conferring “with respect to wages, hours, and other terms and conditions of employment,” § 8 (d) of the Act goes on to provide in relevant part that “where there is in effect a collective-bargaining contract covering employees in an industry affecting commerce, the duty to bargain collectively shall also mean that no party to such contract shall terminate or modify such contract” except upon (1) timely notice to the other party, (2) an offer to meet and confer “for the purpose of negotiating a new contract or a contract containing the proposed modifications,” (3) timely notice to the Federal Mediation and Conciliation Service and comparable state or territorial agencies of the existence of a “dispute,” and (4) continuation “in full force and effect [of] ... all the terms and conditions of the existing contract . . . until [its] expiration date . . . .” 21 *184 The Board's trial examiner ruled that the Company’s action in offering retirees a change in their health plan did not amount to a “modification” of the collective-bargaining agreement in violation of § 8 (d), since the pensioners had merely been given an additional option that they were free to accept or decline as they saw fit. *185 The Board rejected that conclusion on the ground that there were several possible ways of adjusting the negotiated plan to the Medicare provisions and the Company “modified” the contract by unilaterally choosing one of them. The Company now urges, in effect, that we adopt the views of the trial examiner. We need not resolve, however, whether there was a “modification” within the meaning of § 8 (d), because we hold that even if there was, a “modification” is a prohibited unfair labor practice only when it changes a term that is a mandatory rather than a permissive subject of bargaining.

Paragraph (4) of § 8 (d), of course, requires that a party proposing a modification continue “in full force and effect ... all the terms and conditions of the existing contract” until its expiration. Viewed in isolation from the res