Mr. Justice Douglas
delivered the opinion of the Court.
This case presents an important question under § 2 of the Sherman Act, which makes it an offense for any person to “monopolize . . . any part of the trade or commerce among the several States.” This is a civil suit brought by the United States against Grinnell Corporation (Grinnell), American District Telegraph Co. (ADT), Holmes Electric Protective Co. (Holmes) and Automatic Fire Alarm Co. of Delaware (AFA). The District Court held for the Government and entered a decree. All parties appeal, the United States because it deems the relief inadequate and the defendants both on the merits and on the relief and on the ground that the District Court denied them a fair trial. We noted probable jurisdiction. 381 U. S. 910.
Grinnell manufactures plumbing supplies and fire sprinkler systems. It also owns 76% of the stock of ADT, 89% of the stock of AFA, and 100% of the stock of Holmes. ADT provides both burglary and fire protection servicesj Holmes provides burglary services alone; AFA supplies only fire protection service. Each offers a central station service under which hazard-detecting devices installed on the protected premises automatically transmit an electric signal to a central station. The central station is manned 24 hours a day. Upon receipt of a signal, the central station, where appropriate, dispatches guards to the protected premises and notifies the police or fire department direct. There are other forms of protective services. But the record shows that subscribers to accredited central station service (i. e., that approved by the insurance underwriters) receive reductions in their insurance premiums that are substantially greater than the reduction received by the users of other kinds of protection service. In 1961 accredited companies in the central station service business grossed $65,000,000. ADT, Holmes, and AFA are the three largest companies in the business in terms of revenue: ADT (with 121 central stations in 115 cities) has 73% of the business; Holmes (with 12 central stations in three large cities) has 12.5%; AFA (with three central stations in three large cities) has 2%. Thus the three companies that Grinnell controls have over 87% of the business.
Over the years ADT purchased the stock or assets of 27 companies engaged in the business of providing burglar or fire alarm services. Holmes acquired the stock or assets of three burglar alarm companies in New York City using a central station. Of these 30, the officials of seven agreed not to engage in the protective service business in the area for periods ranging from five years to permanently. After Grinnell acquired control of the other defendants, the latter continued in their attempts to acquire central station companies — offers being made to at least eight companies between the years 1955 and 1961, including four of the five largest nondefendant companies in the business. When the present suit was filed, each of those defendants had outstanding an offer to purchase one of the four largest nondefendant companies.
In 1906, prior to the affiliation of ADT and Holmes, they made a written agreement whereby ADT transferred to Holmes its burglar alarm business in a major part of the Middle Atlantic States and agreed to refrain forever from engaging in that business in that area, while Holmes transferred to ADT its watch signal business and agreed to limit its activities to burglar alarm service and night watch service for financial institutions. While this agreement was modified several times and terminated in 1947, in 1961 Holmes still restricted its business to burglar alarm service and operated only in those areas which had been allocated to it under the 1906 agreement. Similarly, ADT continued to refrain from supplying burglar alarm service in those areas earlier allocated to Holmes.
In 1907 Grinnell entered into a series of agreements with the other defendant companies and with Automatic Fire Protection Co. to the following effect:
AFA received the exclusive right to provide central station sprinkler supervisory and waterflow alarm and automatic fire alarm service in New York City, Boston and Philadelphia, and agreed not to provide burglar alarm service in those cities or central station service elsewhere in the United States.
Automatic Fire Protection Co. obtained the exclusive right to provide central station sprinkler supervisory and waterflow alarm service everywhere else in the United States except for the three cities in which AFA received that exclusive right, and agreed not to engage in burglar alarm service.
ADT received the exclusive right to render burglar alarm and nightwatch service throughout the United States. (Under ADT’s 1906 agreement with Holmes, however, it could not provide burglar alarm services in the areas for which it had given Holmes the exclusive right to do so.) It agreed not to furnish sprinkler supervisory and waterflow alarm service anywhere in the country and not to- furnish automatic fire alarm service in New York City, Boston or Philadelphia (the three cities allocated to AFA). ADT agreed to connect to its central stations the systems installed by AFA and Automatic.
Grinnell agreed to furnish and install all sprinkler supervisory and waterflow alarm actuating devices used in systems that AFA and Automatic would install, and otherwise not to engage in the central station protection business.
AFA and Automatic received 25% of the revenue produced by the sprinkler supervisory waterflow alarm service which they provided in their respective territories; ADT and Grinnell received 50% and 25%, respectively, of the revenue which resulted from such service. The agreements were to continue until February 1954.
The agreements remained substantially unchanged until 1949 when ADT purchased all of Automatic Fire Protection Co.’s rights under it for $13,500,000. After these 1907 agreements expired in 1954, AFA continued to honor the prior division of territories; and ADT and AFA entered into a new contract providing for the continued sharing of revenues on substantially the same basis as before. In 1954 Grinnell and ADT renewed an agreement with a Rhode Island company which received the exclusive right to render central station service within Rhode Island at prices no lower than those of ADT and which agreed to use certain equipment supplied by Grinnell and ADT and to share its revenues with those companies. ADT had an informal agreement with a competing central station company in Washington, D. C., “that we would not solicit each other’s accounts.”
ADT over the years reduced its minimum basic rates to meet competition and renewed contracts at substantially increased rates in cities where it had a. monopoly of accredited central station service. ADT threatened retaliation against firms that contemplated inaugurating central station service. And the record indicates that, in contemplating opening a new central station, ADT officials frequently stressed that such action would deter their competitors from opening a new station in that area.
The District Court found that the defendant companies had committed per se violations of § 1 of the Sherman Act as well as § 2 and entered a decree. 236 F. Supp. 244.
I.
The offense of monopoly under § 2 of the Sherman Act has two elements: (1) the possession of monopoly power in the relevant market and (2) the willful acquisition or maintenance of that power as distinguished from growth or development as a consequence of a superior product, business acumen, or historic accident. We shall see that this second ingredient presents no major problem here, as what was done in building the empire was done plainly and explicitly for a single purpose. In United States v. du Pont & Co., 351 U. S. 377, 391, we defined monopoly power as “the power to control prices or exclude competition.” The existence of such power ordinarily may be inferred from the predominant share of the market. In American Tobacco Co. v. United States, 328 U. S. 781, 797, we said that “over two-thirds of the entire domestic field of cigarettes, and . . . over 80% of the field of comparable cigarettes” constituted “a substantial monopoly.” In United States v. Aluminum Co. of America, 148 F. 2d 416, 429, 90% of the market constituted monopoly power. In the present case, 87% of the accredited central station service business leaves no doubt that the congeries of these defendants have monopoly power — power which, as our discussion of the record indicates, they did not hesitate to wield — if that business is the relevant market. The only remaining question therefore is, what is the relevant market?
In case of a product it may be of such a character that substitute products must also be considered, as customers may turn to them if there is a slight increase in the price of the main product. That is the teaching of the du Pont. case {supra, at 395, 404), viz., that commodities reasonably interchangeable make up that “part” of trade or commerce which § 2 protects against monopoly power.
The District Court treated the entire accredited central station service business as a single market and we think it was justified in so doing. Defendants argue that the different central station services offered are so diverse that they cannot under du Pont be lumped together to make up the relevant market. For example, burglar alarm services are not interchangeable with fire alarm services. They further urge that du Pont requires that protective services other than those of the central station variety be included in the market definition.
But there is here a single use, i. e., the protection of property, through a central station that receives signals. It is that service, accredited, that is unique and that competes with all the other forms of property protection. We see no barrier to combining in a single market a number of different products or services where that combination reflects commercial realities. To repeat, there is here a single basic service — the protection of property through use of a central service station — that must be compared with all other forms of property protection.
In § 2' cases under the Sherman Act, as in § 7 cases under the Clayton Act (Brown Shoe Co. v. United States, 370 U. S. 294, 325) there may be submarkets that are separate economic entities. We do not pursue that question here. First, we deal with services, not with products; and second, we conclude that the accredited central station is a type of service that makes up a relevant market and that domination or control of it makes out a monopoly of a “part” of trade or commerce within the meaning of § 2 of the Sherman Act. The defendants have not made out a case for fragmentizing the types of services into lesser units.
Burglar alarm service is in a sense different from fire alarm service; from waterflow alarms; and so on. But it would be unrealistic on this record to break down the market into the various kinds of central station protective services that are available. Central station companies recognize that to compete effectively, they must offer all or nearly all types of service. The different forms of accredited central station service are provided from a single office and customers utilize different services in combination. We held in United States v. Philadelphia Nat. Bank, 374 U. S. 321, 356, that “the cluster” of services denoted by the term “commercial banking” is “a distinct line of commerce.” There is, in our view, a comparable cluster of services here. That bank case arose under § 7 of the Clayton Act where the question was whether the effect of a merger “in any line of commerce” may be “substantially to lessen competition.” We see no reason to differentiate between “line” of commerce in the context of the Clayton Act and “part” of commerce for purposes of the Sherman Act. See United States v. First Nat. Bank & Trust Co., 376 U. S. 665, 667-668. In the § 7 national bank case just mentioned, services, not products in the mercantile sense, were involved. In our view the lumping together of various kinds of services makes for the appropriate market here as it did in the § 7 case.
There are, to be sure, substitutes for the accredited central station service. But none of them appears to operate on the same level as the central station service so as to meet the interchangeability test of the du Pont case. Nonautomatic and automatic local alarm systems appear on this record to have marked differences, not the low degree of differentiation required of substitute services as well as substitute articles.
Watchman service is far more costly and less reliable. Systems that set off an audible alarm at the site of a fire or burglary are cheaper but often less reliable. They may be inoperable without anyone’s knowing it. Moreover, there is a risk that the íocal ringing of an alarm will not attract the needed attention and help. Proprietary systems that a customer purchases and operates are available; but they can be used only by a very large business or by government and are not realistic alternatives for most concerns. There are also protective services connected directly to a municipal police or fire department. But most cities with an accredited central station do not permit direct, connected service for private businesses. These alternate services and devices differ, we are told, in utility, efficiency, reliability, responsiveness, and continuity, and the record sustains that position. And, as noted, insurance companies generally allow a greater reduction in premiums for accredited central station service than for other types of protection.
Defendants earnestly urge that despite these differences, they face competition from these other modes of protection. They seem to us seriously to overstate the degree of competition, but we recognize that (as the District Court found) they “do not have unfettered power to control the price of their services . . . due to the fringe competition of other alarm or watchmen services.” 236 F. Supp., at 254. What defendants overlook is that the high degree of differentiation between central station protection ánd the other forms means that for many customers, only central station protection will do. Though some customers may be willing to accept higher insurance rates in favor of cheaper forms of protection, others will, not be willing or able to risk serious interruption to their businesses, even though covered by insurance, and will thus be unwilling to consider anything but central station protection.
The accredited, as distinguished from nonaccredited service, is a relevant part of commerce. Virtually the only central station companies in the status of the non-accredited are those that have not yet been able to meet the standards of the rating bureau. The accredited ones are indeed those that have achieved, in the eyes of underwriters, superiorities that other central stations do not have. The accredited central station is located in a building of approved design, provided with an emergency lighting system and two alternate main power sources, manned constantly by at least a required minimum of operators, provided with a direct line to fire headquarters and, where possible, a direct line to a police station; and equipped with all the devices, circuits and equipment meeting the requirements of the underwriters. These standards are important as insurance carriers often require accredited central station service as a condition to writing insurance. There is indeed evidence that customers consider the unaccredited service as inferior.
We also agree with the District Court that the geographic market for the accredited central station service is national. The activities of an individual station are in a sense local as it serves, ordinarily, only that area which is within a radius of 25 miles. But the record amply supports the conclusion that the business of providing such a service is operated on a national level. There is national planning. The agreements we have discussed covered activities in many States. The inspection, certification and rate-making is largely by national insurers. The appellant ADT has a national schedule of prices, rates, and terms, though the rates may be varied to meet local conditions. It deals with multistate businesses on the basis of nationwide contracts. The manufacturing business of ADT is interstate. The fact that Holmes is more nearly local than the others does not save it, for it is part and parcel of the combine presided over and controlled by Grinnell.
As the District Court found, the relevant market for determining whether the defendants have monopoly power is not the several local areas which the individual stations serve, but the broader national market that reflects the reality of the way in which they built and conduct their business.
We have said enough about the great hold that the defendants have on this market. The percentage is so high as to justify the finding of monopoly. And, as the facts already related indicate, this monopoly was achieved in large part by unlawful and exclusionary practices. The restrictive agreements that pre-empted for each company a segment of the market where it was free of competition of the others were one device. Pricing practices that contained competitors were another. The acquisitions by Grinnell of ADT, AFA, and Holmes were still another. Grinnell long faced a problem of competing with ADT. That was one reason it acquired AFA and Holmes. Prior to settlement of its dispute and controversy with ADT, Grinnell prepared to go into the central station service business. By acquiring ADT in 1953, Grinnell eliminated that alternative. Its control of the three other defendants eliminated any possibility of an outbreak of competition that might have occurred when the 1907 agreements terminated. By those acquisitions it perfected the monopoly power to exclude competitors and fix prices.
II.
The final decree enjoins the defendants in general terms from restraining trade or attempting or conspiring to restrain trade in this particular market, from further monopolizing, and attempting or conspiring to monopolize. The court ordered the alarm companies to file with the Department of Justice standard lists of prices and terms and every quotation to customers that deviated from those lists and enjoined the defendants from acquiring stock, assets, or business of any enterprise in the market. Grinnell was ordered to file, not later than April 1, 1966, a plan of divestiture of its stock in each of the other defendant companies. It was given the option either to sell the stock or distribute it to its stockholders or combine or vary those methods. The court further enjoined any of the defendants from employing in any capacity the President and Chairman of the Board of Grinnell, James D. Fleming. Both the Government and the defendants challenge aspects of the decree.
We start from the premise that adequate relief in a monopolization case should put an end to the combination and deprive the defendants of any of the benefits of the illegal conduct, and break up or render impotent the monopoly power found to be in violation of the Act. That is the teaching of our cases, notably Schine Theatres v. United States, 334 U. S. 110, 128-129.
We largely agree with the Government’s views on the relief aspect of the case. We start with ADT, which presently does 73% of the business done by accredited central stations throughout the country. It is indeed the keystone of the defendants’ monopoly power. The mere dissolution of the combination through the divestiture by Grinnell of its interests in the other companies does not reach the root of the evil. In 92 of the 115 cities in which ADT operates there are no other accredited central stations. Perhaps some cities could not support more than one. Defendants recognized prior to trial that at least 13 cities can; the Government urged divestiture in 48 cities. That there should be some divestiture on the part of ADT seems clear; but the details of such divestiture must be determined by the District Court as the matter cannot be resolved on this record.
Two of the means by which ADT acquired and maintained its large share of the market are the requirement that subscribers sign five-year contracts and the retention by ADT of title to the protective services equipment installed on a subscriber’s premises. On this record it appears that these practices constitute substantial barriers to competition and that relief against them is appropriate. The pros and cons are argued with considerable vehemence here. Again, we cannot resolve them on this record. The various aspects of this controversy must be explored by the District Court and suitable protective provisions included in the decree that deprive these two devices of the coercive power that they apparently have had towards restraining competition and creating a monopoly.
The Government proposed that the defendants be required to sell, on nondiscriminatory terms, any devices manufactured by them for use in furnishing central station service. It seems clear that if the competitors are to be able to compete effectively for the existing customers of the defendants when the present service contracts expire, they must be assured of replacement parts to maintain those systems.
The Government urges visitation rights, that is, requiring reports, examining documents, and interviewing company personnel, a relief commonly granted for the purpose of determining whether a defendant has complied with an antitrust decree. See United States v. United States Gypsum Co., 340 U. S. 76, 95. The District Court gave no explanation for its refusal to grant this relief. It is so important and customary a provision that the District Court should reconsider it.
Defendants urge and the Government concedes that the barring of Mr. Fleming from the employment of any of the defendants is unduly harsh and quite unnecessary on this record. While relief of that kind may be appropriate where the predatory conduct is conspicuous, we cannot see that any such case was made out on this record.
The Government objects, as do the defendants, to the broad and generalized terms of the restraining order. They properly point out, as we emphasized in Schine Theatres v. United States, supra, at 125-126, that the precise practices found to have violated the Act should be specifically enjoined. On remand we suggest that that course be taken.
The defendants object to the requirements that Grin-nell divest itself of its hqldings in the three alarm company defendants, but' we think that provision is wholly justified. Dissolution of the combination is essential as indicated by many of our cases, starting with Standard Oil Co. v. United States, 221 U. S. 1, 78. The defendants object to that portion of the decree that bars them from acquiring interests in firms in the accredited central station business. But since acquisition was one of the methods by which the defendants acquired their market power and was the method by which Grinnell put the combination together, an injunction against the repetition of the practice seems fully warranted. The defendants further object to the requirement in the decree that the alarm company defendants report to the Department of Justice any deviation they make from their list prices. We make no comment on that because in view of the other extensive changes necessary in the decree, the District Court might well deem it to be unnecessary in the fashioning of the new decree. In other words, we leave that matter open, to rest finally in the discretion of the District Court.
III.
The defendants contend that Judge Wyzanski, who tried the case, was personally biased and prejudiced and should have been disqualified from sitting in the case, and that he denied them a fair trial. We think this point is without merit.
The complaint was filed in April 1961, the answers in July 1961. Shortly thereafter extensive taking of depositions began. The District Court in January 1963 directed that no depositions be taken after September 1, 1963. In response to an inquiry from the court both sides suggested that the trial be set no earlier than January 1964.
At a pretrial conference in December 1963, government counsel told the court that the parties had been trying to reach agreement on a consent decree but were far apart and asked how the court would like to handle the presentation of the evidence in the event a settlement was not reached. Grinnell’s lawyer suggested that the next appropriate procedure would be a pretrial on the question of relief — a suggestion that the District Court construed as an invitation to the court to discuss the relief apart from the merits. The Government objected. The court then asked for a brief from each side setting forth its views on relief if the Government prevailed on the merits. In response to the court’s statement that “as I understand it, you want to find out what kind of relief I would be likely to allow if the government’s case stood virtually uncontradicted,” Grinnell’s counsel replied: “That is what I had in mind, your Honor, yes.” Thereupon the court set a day for such a hearing. At the next pretrial conference Grinnell’s counsel stated that “if your Honor would indicate the relief that might be appropriate in this case that would help both sides to come to a better understanding.”
Then the following colloquy occurred:
“The Court. I don’t think it would help very much.
“Mr. McInerney. Well, your Honor, I think it would help both the plaintiff and the defendants to know what is really at stake here in this trial.
“The Court. I assure you that you would not be helped by anything I would say. You would do better to get together with the government rather than run the risk of what I would say from what I have seen. Let me just assure you of that. . . .”
The case was then set for trial on June 15,1964. When Grinnell’s counsel sought to argue further, the court stated: “There is no use in discussing it with me. I have read enough to know that if I have to decide this case on what I have seen from the government you will not be in a position at this stage to agree to it.”
On June 3, 1964, defendants argued for a postponement of the trial, saying they needed more time. The court denied the motion. Then they argued that the relief issues to be tried be limited to those raised by the pleadings so as to eliminate what they considered to be extraneous issues raised by the Government. To that the court replied:
“I can’t understand frankly why you don’t realize that you have forced me to look at the documents in this case, which I dislike doing in advance of trial. You have invited me, therefore, into what I regard as, from your point of view, a rather undesirable situation. I think I made that clear at the beginning. I have told you that, forced by you to look, my views are more extreme than those of the government; and I have also made you realize that if I am required to make Findings and reach Conclusions I am opening up third-party suits that will make, in view of the size of the industry, the percentage of people involved higher than in the electrical cases.”
Shortly thereafter defendants filed a motion for the disqualification of Judge Wyzanski on the grounds of personal bias and prejudice.
The alleged bias and prejudice to be disqualifying must stem from an extrajudicial source and result in an opinion on the merits on some basis other than what the judge learned from his participation in the case. Berger v. United States, 255 U. S. 22, 31. Any adverse attitudes that Judge Wyzanski evinced toward the defendants were based on his study of the depositions and briefs which the parties had requested him to make. What he said reflected no more than his view that, if the facts were as the Government alleged, stringent relief was called for.
During the trial he repeatedly stated that he had not made up his mind on the merits. During the trial he ruled certain evidence to be irrelevant to the issues and when the lawyer persisted in offering it Judge Wyzanski said, “Maybe you will persuade somebody else. And if you think so, all right. I just assure you it is a great ceremonial act, as far as I am concerned.” We do not read this statement as manifesting a closed mind on the merits of the case but consider it merely a terse way of repeating the previously stated ruling that this particular evidence was irrelevant.
We have examined all the other claims of the defendants made against Judge Wyzanski and find that the claim of bias and prejudice is not made out. Our discussion of the relief which he granted shows indeed that he was, in several critical respects, too lenient with those who now charge him with bias and prejudice.
The judgment below is affirmed except as to the decree. We remand for further hearings on the nature of the relief consistent with the views expressed herein.
It is so ordered.
26 Stat. 209, as.amended, 15 U. S. C. §2 (1964 ed.).
Expediting Act § 2, 32 Stat. 823, as amended, 15 U. S. C. § 29 (1964 ed.); United States v. Loew’s, Inc., 371 U. S. 38.
These are the record figures. Since the time of the trial, Grinnell’s holdings have increased. Counsel for Grinnell has advised this Court that Grinnell now holds 80% of ADT’s stock and 90% of the stock of AFA.
Among the various central station services offered are the following:
(1) automatic burglar alarms;
(2) automatic fire alarms;
(3) sprinkler supervisory service (any malfunctions in the fire sprinkler system — e. g., changes in water pressure, dangerously low water temperatures, etc. — are reported to the central station); and
(4) watch signal service (night watchmen, by operating a key-triggered device on the protected premises, indicate to the central station that they are making their rounds and that all is well; the failure of a watchman to make his electrical report alerts the central station that something may be amiss).
In 1959, ADT complained that AFA’s share of the revenues was excessive. AFA replied, in a letter to the president of Grinnell (which by that time controlled both ADT and AFA), that its share was just compensation for its continued observance of the service and territorial restrictions: “[T]he geographic restrictions placed upon us plus the requirement that we confine our activities to sprinkler and fire alarm services exclusively, since 1907 and presumably into the future, has definitely retarded our expansion in the past to the benefit of ADT growth. . . . [AFA’s] contribution must also include the many things that helped make ADT big.” (Emphasis added.)
Thus, of the 38 nondefendant firms operating a central service station protective service in the United States in 1961, 24 offered all of the following services: automatic fire alarm; waterflow alarm and sprinkler supervision; watchman’s reporting and manual fire alarm; and burglar alarm. Of the other firms, 11 provided no watchman’s reporting and manual fire alarm service; six provided no automatic fire alarm service; and two offered no sprinkler supervisory and waterflow alarm service. Moreover, of the 14 firms not providing the full panoply of services, 10 lacked only one of the above-described services. Appellant ADT’s assertion that “very few accredited central stations furnish the full variety of services” is flatly contradicted by the record.
Since the record clearly shows that this monopoly power was consciously acquired, we have no reason to reach the further position of the District Court that once monopoly power is shown to exist, the burden is on the defendants to show that their dominance is due to skill, acumen, and the like.
Although the Government originally urged that the decree was inadequate as to divestiture in -that it permitted Grinnell to distribute the stock of the other companies to Grinnell’s shareholders, it has abandoned that point in this Court.
Specifically, the areas of disagreement are: (1) Defendants urge that barring them from offering five-year contracts would put them at a competitive disadvantage vis-á-vis nondefendant firms; the Government responds that since they violated the law, they may properly be subjected to restrictions not borne by others. See United States v. Bausch & Lomb Co., 321 U. S. 707, 723-724. (2) Some customers of defendants may wish to have long-term contracts; the Government responds that this may be explored on remand. (3) There is some dispute as to whether, if the central station company cannot retain title to the equipment it installs, the insurance companies will accredit the system. This, too, is a proper subject for inquiry on remand.
Prior to trial, the defendants agreed that this would be an appropriate provision in a decree were the Government to prevail in all its claims of antitrust violations. Although defendants now maintain that this pretrial discussion was “settlement talk,” that earlier concession is a relevant factor that the District Judge can properly take into account on remand.
This provision, too, gained pretrial acceptance. See n. 10, supra.
28 U. S. C. § 144 (1964 ed.) provides in relevant part:
“Whenever a party to any proceeding in a district court makes and files a timely and sufficient affidavit that the judge before whom the matter is pending has a personal bias or prejudice either against him or in favor of any adverse party, such judge shall proceed no further therein, but another judge shall be assigned to hear such proceeding.”
Judge Wyzanski referred the question of his disqualification to Chief Judge Woodbury of the Court of Appeals for the First Circuit who after hearing oral argument held that no case of bias and prejudice had been made out under § 144.