delivered the opinion of the Court.
This case presents the question of whether a United States District Court may properly direct a telephone company to provide federal law enforcement officials the facilities and technical assistance necessary for the implementation of its order authorizing the use of pen registers1 to investigate offenses which there was probable cause to believe were being committed by means of the telephone.
I
On March 19, 1976, the United States District Court for the Southern District of New York issued an order authorizing agents of the Federal Bureau of Investigation (FBI) to install and use pen registers with respect to two telephones and directing the New York Telephone Co. (Company) to furnish the FBI “all information, facilities and technical assistance” necessary to employ the pen registers unobtrusively. The FBI was ordered to compensate the Company at prevailing rates for any assistance which it furnished. App. 6-7. The order was issued on the basis of an affidavit sub*162mitted by an FBI agent which stated that certain individuals were conducting an illegal gambling enterprise at 220 East I4th Street in New York City and that, on the basis of facts set forth therein, there was probable cause to believe that two telephones bearing different numbers were being used at that address in furtherance of the illegal activity. Id., at 1-5. The District Court found that there was probable cause to conclude that an illegal gambling enterprise using the facilities of interstate commerce was being conducted at the East 14th Street address in violation of 18 U. S. C. §§ .371 and 1952, and that the two telephones had been, were currently being, and would continue to be used in connection with those offenses. Its order authorized the FBI to operate the pen registers with respect to the two telephones until knowledge of the numbers dialed led to the identity of the associates and confederates of those believed to be conducting the illegal operation or for 20 days, “whichever is earlier.”
The Company declined to comply fully with the court order. It did inform the FBI of the location of the relevant “appearances,” that is, the places where specific telephone lines emerge from the sealed telephone cable. In addition, the Company agreed to identify the relevant “pairs,” or the specific pairs of wires that constituted the circuits of the two telephone lines. This information is required to install a pen register. The Company, however, refused to lease lines to the FBI which were needed to install the pen registers in an unobtrusive fashion. Such lines were required by the FBI in order to install the pen registers in inconspicuous locations away from the building containing the telephones. A “leased line” is an unused telephone line which makes an “appearance” in the same terminal box as the telephone line in connection with which it is desired to install a pen register. If the leased line is connected to the subject telephone line, the pen register can then be installed on the leased line at a remote location and be monitored from that point. The *163Company, instead of providing the leased lines, which it conceded that the court’s order required it to do, advised the FBI to string cables from the “subject apartment” to another location where pen registers could be installed. The FBI determined after canvassing the neighborhood of the apartment for four days that there was no location where it could string its own wires and attach the pen registers without alerting the suspects,2 in which event, of course, the gambling operation would cease to function. App. 15-22.
On March 30, 1976, the Company moved in the District Court to vacate that portion of the pen register order directing it to furnish facilities and technical assistance to the FBI in connection with the use of the pen registers on the ground that such a directive could be issued only in connection with a wiretap order conforming to the requirements of Title III of the Omnibus Crime Control and Safe Streets Act of 1968, 18 U. S. C. §§2510-2520 (1970 ed. and Supp. V). It contended that neither Fed. Rule Crim. Proc. 41 nor the All Writs Act, 28 U. S. C. § 1651 (a), provided any basis for such an order. App. 10-14. The District Court ruled that pen registers are not governed by the proscriptions of Title III because they are not devices used to intercept oral communications. It concluded that it had jurisdiction to authorize the installation of the pen registers upon a showing of probable cause and that both the All Writs Act and its inherent powers provided authority for the order directing the Company to assist in the installation of the pen registers.
On April 9, 1976, after the District Court and the Court of Appeals denied the Company’s motion to stay the pen register order pending appeal, the Company provided the leased lines.3
*164The Court of Appeals affirmed in part and reversed in part, with one judge dissenting on the ground that the order below should have been affirmed in its entirety. Application of United States in re Pen Register Order, 538 F. 2d 956 (CA2 1976). It agreed with the District Court that pen registers do not fall within the scope of Title III and are not otherwise prohibited or regulated by statute. The Court of Appeals also concluded that district courts have the power, either inherently or as a logical derivative of Fed. Crim. Proc. 41, to authorize pen register surveillance upon an adequate showing of probable cause. The majority held, however, that the District Court abused its discretion in ordering the Company to assist in the installation and operation of the pen registers. It assumed, arguendo, that “a district court has inherent discretionary authority or discretionary power under the All Writs Act to compel technical assistance by the Telephone Company,” but concluded that “in the absence of specific and properly limited Congressional action, it was an abuse of discretion for the District Court to order the Telephone Company to furnish technical assistance.” 538 F. 2d, at 961.4 The majority expressed concern that “such an order could establish a most undesirable, if not dangerous and unwise, precedent for the authority of federal courts to impress unwilling aid on private third parties” and that “there is no assurance that the court will always be able to protect '[third parties] from excessive or overzealous Government activity or compulsion.” Id., at 962-963.5
*165We granted the United States’ petition for certiorari challenging the Court of Appeals’ invalidation of the District Court’s order against respondent.6 429 U. S. 1072.
II
We first reject respondent’s contention, which is renewed here, that the District Court lacked authority to order the Company to provide assistance because the use of pen registers may be authorized only in conformity with the procedures set forth in Title III7 for securing judicial authority to inter*166cept wire communications.8 Both the language of the statute and its legislative history establish beyond any doubt that pen registers are not governed by Title III.9
Title III is concerned only with orders “authorizing or approving the interception of a wire or oral communication . . . .” 18 U. S. C. §2518(1) (emphasis added).10 Congress defined “intercept” to mean “the aural acquisition of the contents of any wire or oral communication through the use of any electronic, mechanical, or other device.” 18 U. S. C. *167§ 2510 (4) (emphasis added). Pen registers do not “intercept” because they do not acquire the “contents” of communications, as that term is defined by 18 U. S. C. § 2510 (8).11 Indeed, a law enforcement official could not even determine from the use of a pen register whether a communication existed. These devices do not hear sound. They disclose only the telephone numbers that have been dialed — a means of establishing communication. Neither the purport of any communication between the caller and the recipient of the call, their identities, nor whether the call was even completed is disclosed by pen registers. Furthermore, pen registers do not accomplish the “aural acquisition” of anything. They decode outgoing telephone numbers by responding to changes in electrical voltage caused by the turning of the telephone dial (or the pressing of buttons on pushbutton telephones) and present the information in a form to be interpreted by sight rather than by hearing.12
The legislative history confirms that there was no congressional intent to subject pen registers to the requirements of Title III. The Senate Report explained that the definition of “intercept” was designed to exclude pen registers:
“Paragraph 4 [of § 2510] defines 'intercept’ to include the aural acquisition of the contents of any wire or oral communication by any electronic, mechanical, or other device. Other forms of surveillance are not within the proposed legislation. . . . The proposed legislation is not designed to prevent'the tracing of phone calls. The use of a 'pen register/ for example, would be permissible. But see United States v. Dote, 371 F. 2d 176 (7th 1966). The proposed legislation is intended to protect the privacy of the communication itself and not the means of *168communication.” S. Rep. No. 1097, 90th Cong., 2d Sess., 90 (1968).13
It is clear that Congress did not view pen registers as posing a threat to privacy of the same dimension as the interception of oral communications and did not intend to impose Title III restrictions upon their use.
Ill
We also agree with the Court of Appeals that the District Court had power to authorize the installation of the pen registers.14 It is undisputed that the order in this case was predicated upon a proper finding of probable cause, and no claim is made that it was in any way inconsistent with the *169Fourth Amendment. Federal Rule Crim. Proc. 41 (b) authorizes the issuance of a warrant to:
“search for and seize any (1) property that constitutes evidence of the commission of a criminal offense; or (2) contraband, the fruits of crime, or things otherwise criminally possessed; or (3) property designed or intended for use or which is or has been used as the means of committing a criminal offense.”
This authorization is broad enough to encompass a “search” designed to ascertain the use which is being made of a telephone suspected of being employed as a means of facilitating a criminal venture and the “seizure” of evidence which the “search” of the telephone produces. Although Rule 41 (h) defines property “to include documents, books, papers and any other tangible objects,” it does not restrict or purport to exhaustively enumerate all the items which may be seized pursuant to Rule 41.15 Indeed, we recognized in Katz v. United States, 389 U. S. 347 (1967), which held that telephone conversations were protected by the Fourth Amendment, that Rule 41 is not limited to tangible items but is sufficiently flexible to include within its scope electronic intrusions authorized upon a finding of probable cause. 389 U. S., at 354-356, and n. 16.16 See also Osborn v. United States, 385 U. S. 323, 329-331 (1966).
*170Our conclusion that Rule 41 authorizes the use of pen registers under appropriate circumstances is supported by Fed. Rule Crim. Proc. 57 (b), which provides: “If no procedure is specifically prescribed by rule, the court may proceed in any lawful manner not inconsistent with these rules or with any applicable statute.” 17 Although we need not and do not decide whether Rule 57 (b) by itself would authorize the issuance of pen register orders, it reinforces our conclusion that Rule 41 is sufficiently broad to include seizures of intangible items such as dial impulses recorded by pen registers as well as tangible items.
Finally, we could not hold that the District Court lacked any power to authorize the use of pen registers without defying the congressional judgment that the use of pen registers “be permissible.” S. Rep. No. 1097, supra, at 90. Indeed, it would be anomalous to permit the recording of conversations by means of electronic surveillance while prohibiting the far lesser intrusion accomplished by pen registers. Congress intended no such result. We are unwilling to impose it in the absence of some showing that the issuance of such orders would be inconsistent with Rule 41. Cf. Rule 57 (b), supra.18
*171IV
The Court of Appeals held that even though the District Court had ample authority to issue the pen register warrant and even assuming the applicability of the All Writs Act, the order compelling the Company to provide technical assistance constituted an abuse of discretion. Since the Court of Appeals conceded that a compelling case existed for requiring the assistance of the Company and did not point to any fact particular to this case which would warrant a finding of abuse of discretion, we interpret its holding as generally barring district courts from ordering any party to assist in the installation or operation of a pen register. It was apparently concerned that sustaining the District Court's order would authorize courts to compel third parties to render assistance without limitation regardless of the burden involved and pose a severe threat to the autonomy of third parties who for whatever reason prefer not to render such assistance. Consequently the Court of Appeals concluded that courts should not *172embark upon such a course without specific legislative authorization. We agree that the power of federal courts to impose duties upon third parties is not without limits; unreasonable burdens may not be imposed. We conclude, however, that the order issued here against respondent was clearly authorized by the All Writs Act and was consistent with the intent of Congress.19
The All Writs Act provides:
“The Supreme Court and all courts established by Act of Congress may issue all writs necessary or appropriate in aid of their respective jurisdictions and agreeable to the usages and principles of law.” 28 U. S. C. § 1651 (a).
The assistance of the Company was required here to implement a pen register order which we have held the District Court was empowered to issue by Rule 41. This Court has repeatedly recognized the power of a federal court to issue such commands under the All Writs Act as may be necessary or appropriate to effectuate and prevent the frustration of orders it has previously issued in its exercise of jurisdiction otherwise obtained: “This statute has served since its inclusion, in substance, in the original Judiciary Act as a ‘legislatively approved source of procedural instruments designed to achieve “the rational ends of law.” ’ ” Harris v. Nelson, 394 U. S. 286, 299 (1969), quoting Price v. Johnston, 334 U. S. 266, 282 (1948). Indeed, “[ujnless appropriately confined by *173Congress, a federal court may avail itself of all auxiliary writs as aids in the performance of its duties, when the use of such historic aids is calculated in its sound judgment to achieve the ends of justice entrusted to it.” Adams v. United States ex rel. McCann, 317 U. S. 269, 273 (1942).
The Court has consistently applied the Act .flexibly in conformity with these principles. Although § 262 of the Judicial Code, the predecessor to § 1651, did not expressly authorize courts, as does § 1651, to issue writs “appropriate” to the proper exercise of their jurisdiction but only “necessary” writs, Adams held that these supplemental powers are not limited to those situations where it is “necessary” to issue the writ or order “in the sense that the court could not otherwise physically discharge its appellate duties.” 317 U. S., at 273. In Price v. Johnston, supra, § 262 supplied the authority for a United States Court of Appeals to issue an order commanding that a prisoner be brought before the court for the purpose of arguing his own appeal. Similarly, in order to avoid frustrating the “very purpose” of 28 U. S. C. § 2255, § 1651 furnished the District Court with authority to order that a federal prisoner be produced in court for purposes of a hearing. United States v. Hayman, 342 U. S. 205, 220-222 (1952). The question in Harris v. Nelson, supra, was whether, despite the absence of specific statutory authority, the District Court could issue a discovery order in connection with a habeas corpus proceeding pending before it. Eight Justices agreed that the district courts have power to require discovery when essential to render a habeas corpus proceeding effective. The Court has also held that despite the absence of express statutory authority to do so, the Federal Trade Commission may petition for, and a Court of Appeals may issue, pursuant to § 1651, an order preventing a merger pending hearings before the Commission to avoid impairing or frustrating the Court of Appeals’ appellate jurisdiction. FTC v. Dean Foods Co., 384 U. S. 597 (1966).
*174The power conferred by the Act extends, under appropriate circumstances, to persons who, though not parties to the original action or engaged in wrongdoing, are in a position to frustrate the implementation of a court order or the proper administration of justice, Mississippi Valley Barge Line Co. v. United States, 273 F. Supp. 1, 6 (ED Mo. 1967), summarily aff’d, 389 U. S. 579 (1968); Board of Education v. York, 429 F. 2d 66 (CA10 1970), cert. denied, 401 U. S. 954 (1971), and encompasses even those who have not taken any affirmative action to hinder justice. United States v. McHie, 196 F. 586 (ND Ill. 1912); Field v. United States, 193 F. 2d 92, 95-96 (CA2), cert. denied, 342 U. S. 894 (1951).20
Turning to the facts of this case, we do not think that the Company was a third party so far removed from the underlying controversy that its assistance could not be permissibly compelled. A United States District Court found that there was probable cause to believe that the Company’s facilities were being employed to facilitate a criminal enterprise on a continuing basis. For the Company, with this knowledge, to refuse to supply the meager assistance required by the FBI in its efforts to put an end to this venture threatened obstruction of an investigation which would determine whether the Company’s facilities were being lawfully used. Moreover, it can hardly be contended that the Company, a highly regulated public utility with a duty to serve the public,21 had a substantial interest in not providing assistance. Certainly the use of pen registers is by no means offensive to it. The Company concedes that it regularly employs such devices without court order for the purposes of checking billing operations, detecting fraud, and *175preventing violations of law.22 It also agreed to supply the FBI with all the information required to install its own pen registers. Nor was the District Court’s order in any way burdensome. The order provided that the Company be fully reimbursed at prevailing rates, and compliance with it required minimal effort on the part of the Company and no disruption to its operations.
Finally, we note, as the Court of Appeals recognized, that without the Company’s assistance there is no conceivable way in which the surveillance authorized by the District Court could have been successfully accomplished.23 The FBI, after an exhaustive search, was unable to find a location where it could install its own pen registers without tipping off the targets of the investigation. The provision of a leased line by the Company was essential to the fulfillment of the purpose— to learn the identities of those connected with the gambling operation — for which the pen register order had been issued.24
*176The order compelling the Company to provide assistance was not only consistent with the Act but also with more recent congressional actions. As established in Part II, supra, Congress clearly intended to permit the use of pen registers by federal law enforcement officials. Without the assistance of the Company in circumstances such as those presented here, however, these devices simply cannot be effectively employed. Moreover, Congress provided in a 1970 amendment to Title III that “fa]n order authorizing the interception of a wire or oral communication shall, upon request of the applicant, direct that a communication common carrier . . . shall furnish the applicant forthwith all information, facilities, and technical assistance necessary to accomplish the interception unobtrusively . . . .” 18 U. S. C. § 2518 (4). In light of this direct *177command to federal courts to compel, upon request, any assistance necessary to accomplish an electronic interception, it would be remarkable if Congress thought it beyond the power of the federal courts to exercise, where required, a discretionary authority to order telephone companies to assist in the installation and operation of pen registers, which accomplish a far lesser invasion of privacy.25 We are convinced that *178to prohibit the order challenged here would frustrate the clear indication by Congress that the pen register is a permissible law enforcement tool by enabling a public utility to thwart a judicial determination that its use is required to apprehend and prosecute successfully those employing the utility’s facilities to conduct a criminal venture. The contrary judgment of the Court of Appeals is accordingly reversed.
So ordered.
A pen. register is a mechanical device that records the numbers dialed on a telephone by monitoring the electrical impulses caused when the dial on the telephone is released. It does not overhear oral communications and does not indicate whether calls are actually completed.
The gambling operation was known to employ countersurveillance techniques. App. 21.
On the same date another United States District Court judge extended the original order of March 19 for an additional 20 days. Id., at 33.
The Court of Appeals recognized that "without [the Company’s] technical aid, the order authorizing the use of a pen register will be worthless. Federal law enforcement agents simply cannot implement pen register surveillance without the Telephone Company’s help. The assistance requested requires no extraordinary expenditure of time or effort by [the Company]; indeed, as we understand it, providing lease or private lines is a relatively simple, routine procedure.” 538 F. 2d, at 961-962.
Judge Mansfield dissented in part on the ground that the District Court possessed a discretionary power under the All Writs Act to direct the *165company to render such assistance as was necessary to implement its valid order authorizing the use of pen registers and that a compelling case had been established for the exercise of discretion in favor of the assistance order. He argued that district court judges could be trusted to exercise their powers under the All Writs Act only in cases of clear necessity and to balance the burden imposed upon the party required to render assistance against the necessity.
Although the pen register surveillance had been completed by the time the Court of Appeals issued its decision on July 13, 1976, this fact does not render the case moot, because the controversy here is one “capable of repetition, yet evading review.” Southern Pacific Terminal Co. v. ICC, 219 U. S. 498, 515 (1911); Roe v. Wade, 410 U. S. 113, 125 (1973). Pen register orders issued pursuant to Fed. Rule Crim. Proc. 41 authorize surveillance only for brief periods. Here, despite expedited action by the Court of Appeals, the order, as.extended, expired six days after oral argument. Moreover, even had the pen register order been stayed pending appeal, the mootness problem would have remained, because the showing of probable cause upon which the order authorizing the installation of the pen registers was based would almost certainly have become stale before review could have been completed. It is also plain, given the Company’s policy of refusing to render voluntary assistance in installing pen registers and the Government’s determination to continue to utilize them,, that the Company will be subjected to similar orders in the future. See Weinstein v. Bradford, 423 U. S. 147, 149 (1975).
The Court of Appeals held that pen register surveillance was subject to the requirements of the Fourth Amendment. This conclusion is not challenged by either party, and we find it unnecessary to consider the matter. The Government concedes that its application for the pen register order did not' conform to the requirements of Title III.
Although neither this issue nor that of the scope of Fed. Rule Crim. Proc. 41 is encompassed within the question posed in the petition for certiorari and the Company has not filed a cross-petition, we have discretion to consider them because the prevailing party may defend a judgment on any ground which the law and the record permit that would not expand the relief it has been granted. Langnes v. Green, 282 U. S. 531, 538-539 (1931); Dandridge v. Williams, 397 U. S. 471, 475 n. 6 (1970). The only relief sought by the Company is that granted by the Court of Appeals: the reversal of the District Court’s order directing it to assist in the installation and operation of the pen registers. The Title III and Rule 41 questions were considered by both the District Court and the Court of Appeals and fully argued here.
Four Justices reached this conclusion in United States v. Giordano, 416 U. S. 505, 553-554 (1974) (Powell, J., joined by Burger, C. J., and Blackmun and Rehnquist, JJ., concurring in part and dissenting in part). The Court’s opinion did not reach the issue since the evidence derived from a pen register was suppressed as being in turn derived from an illegal wire interception. Every Court of Appeals that has considered the matter has agreed that pen registers are not within the scope of Title III. See United States v. Illinois Bell Tel. Co., 531 F. 2d 809 (CA7 1976); United States v. Southwestern Bell Tel. Co., 546 F. 2d 243 (CA8 1976); Michigan Bell Tel. Co. v. United States, 565 F. 2d 385 (CA6 1977); United States v. Falcone, 505 F. 2d 478 (CA3 1974), cert. denied, 420 U. S. 955 (1975); Hodge v. Mountain States Tel. & Tel. Co., 555 F. 2d 254 (CA9 1977); United States v. Clegg, 509 F. 2d 605, 610 n. 6 (CA5 1975).
Similarly, the sanctions of Title III are aimed only at one who “willfully intercepts, endeavors to intercept, or procures any other person to intercept or endeavor to intercept, any wire or oral communication ... .” 18 U. S. C. §2511 (l)(a).
“ ‘Contents’. . . includes any information concerning the identity of the parties to [the] communication or the 'existence, substance, purport, or meaning of [the] communication.”
See 538 P. 2d, at 957.
United States v. Dote, 371 F. 2d 176 (CA7 1966), held that § 605 of the Communications Act of 1934, 47 U. S. C. § 605, which prohibited the interception and divulgence of “any communication” by wire or radio, included pen registers within the scope of its ban. In § 803 of Title III, 82 Stat. 223, Congress amended § 605 by restricting it to the interception of “any radio communication.” Thus it is clear that pen registers are no longer within the scope of § 605. See Korman v. United States, 486 F. 2d 926, 931-932 (CA7 1973). The reference to Dote in the Senate Report is indicative of Congress’ intention not to place restrictions upon their use. We find no merit in the Company’s suggestion that the reference to Dote is merely an oblique expression of Congress’ desire that telephone companies be permitted to use pen registers in the ordinary course of business, as Dote allowed, so long as they are not used to assist law enforcement. Brief for Respondent 16. The sentences preceding the reference to' Dote state unequivocally that pen registers are not within the scope of Title III. In addition, a separate provision of Title III, 18 U. S. C. § 2511 (2) (a) (i), specifically excludes all normal telephone company business practices from the prohibitions of the Act. Congress clearly intended to disavow Dote to the extent that it prohibited the use of pen registers by law enforcement authorities.
The Courts of Appeals that have considered the question have agreed that pen register orders are authorized by Fed. Rule Crim. Proc. 41 or by an inherent power closely akin to it to issue search warrants under circumstances conforming to the Fourth Amendment. See Michigan Bell Tel. Co., supra; Southwestern Bell Tel. Co., supra; Illinois Bell Tel. Co., supra.
Where the definition of a term in Rule 41 (h) was intended to be all inclusive, it is introduced by the phrase “to mean” rather than “to include.” Cf. Helvering, v. Morgan’s, Inc., 293 U. S. 121, 125 n. 1 (1934).
The question of whether the FBI, in its implementation of the District Court’s pen register authorization, complied with all the requirements of Rule 41 is not before us. In Katz, the Court stated that the notice requirement of Rule 41 (d) is not so inflexible as to require invariably that notice be given the person “searched” prior to the commencement of the search. 389 U. S., at 355-356, n. 16. Similarly, it is clear to us that the requirement of Rule 41 (c) that the warrant command that the search be conducted within 10 days of its issuance does not mean that the duration of a pen register surveillance may not exceed 10 days. Thus *170the District Court’s order, which authorized surveillance for a 20-day period, did not conflict with Rule 41.
See United States v. Baird, 414 F. 2d 700, 710 (CA2 1969), cert. denied, 396 U. S. 1005 (1970); Jackson v. United States, 122 U. S. App. D. C. 324, 326, 353 F. 2d 862, 864 (1965); United States v. Remolif, 227 F: Supp. 420, 423 (Nev. 1964); Link v. Wabash R. Co., 370 U. S. 626, 633 n. 8 (1962) (applying the analogous provision of Fed. Rule Civ. Proc. 83).
The dissent argues, post, at 182-184, that Rule 41 (b), as modified following Warden v. Hayden, 387 U. S. 294 (1967), to explicitly authorize searches for any property that constitutes evidence of a crime, falls short of authorizing warrants to “search” for and "seize” intangible evidence. The elimination of the restriction against seizing property that is “mere evidence,” however, has no bearing whatsoever on the scope of the definition of property set forth in Rule 41 (h) which, as the dissent acknowledges, remained unchanged. Moreover, the definition of property set forth in *171Rule 41 (h) is introduced by the phrase, “[t]he term 'property’ is used in this rule to include" (emphasis added), which indicates that it was not intended to be exhaustive. See supra, at 169.
We are unable to comprehend the logic supporting the dissent’s contention, post, at 184-185, that the conclusion of Katz v. United States that Rule 41 was not confined to tangible property did not survive the enactment of Title III and Title IX of the Omnibus Crime Control and Safe Streets Act of 1968, because Congress failed to expand the definition of property contained in Rule 41 (h). There was obviously no need for any such action in light of the Court’s construction of the Rule in Katz. The dissent’s assertion that it “strains credulity” to conclude that Congress intended to permit the seizure of intangibles outside the scope of Title III without its safeguards disregards the congressional judgment that the use of pen registers be permissible without Title III restrictions. Indeed, the dissent concedes that pen registers are not governed by Title III. What “strains credulity” is the dissent’s conclusion, directly contradicted by the legislative history of Title III, that Congress intended to permit the interception of telephone conversations while prohibiting the use of pen registers to obtain much more limited information.
The three other Courts of Appeals which have considered the question reached a different conclusion from the Second Circuit. The Sixth Circuit in Michigan Bell Tel. Co. v. United States, 565 F. 2d 385 (1977), and the Seventh Circuit in United States v. Illinois Bell Tel. Co., 531 F. 2d 809 (1976), held that the Act did authorize the issuance of orders compelling a telephone company to assist in the use of surveillance devices not covered by Title III such as pen registers. The Eighth Circuit found such authority to be part of the inherent power of district courts and “concomitant of the power to authorize pen register surveillance.” United States v. Southwestern Bell Tel. Co., 546 F. 2d, at 246.
See Labette County Comm’rs v. Moulton, 112 U. S. 217, 221 (1884): “[I]t does not follow because the jurisdiction in mandamus [now included in § 1651] is ancillary merely that it cannot be exercised over persons not parties to the judgment sought to be enforced.”
See 47 U. S. C. § 201 (a) and N. Y. Pub. Serv. Law § 91 (McKinney 1955 and Supp. 1977-1978).
Tr. of Oral Arg. 27-28, 40.
The dissent’s attempt to draw a distinction between orders in aid of a court’s own duties and jurisdiction and orders designed to better enable a party to effectuate his rights and duties, post, a.t 189-190, is specious. Courts normally exercise their jurisdiction only in order to protect the legal rights of parties. In Price v. Johnston, 334 U. S. 266 (1948), for example, the production of the federal prisoner in court was required in order to enable him to effectively present his appeal which the court had jurisdiction to hear. Similarly, in Harris v. Nelson, 394 U. S. 286 (1969), discovery was ordered in connection with a habeas corpus proceeding for the purpose of enabling a prisoner adequately to protect his rights. Here, we have held that Fed. Rule Crim. Proc. 41 provided the District Court with power to authorize the FBI to install pen registers. The order issued by the District Court compelling the Company to provide technical assistance was required to prevent nullification of the court’s warrant and the frustration of the Government’s right under the warrant to conduct a pen register surveillance, just as the orders issued in Price and Harris were necessary to protect the rights of prisoners.
We are unable to agree with the Company’s assertion that “it is extraordinary to expect citizens to directly involve themselves in the law *176enforcement process.” Tr. of Oral Arg. 41. The conviction that private citizens have a duty to provide assistance to law enforcement officials when it is required is by no means foreign to our traditions, as the Company apparently believes. See Babington v. Yellow Taxi Corp., 250 N. Y. 14, 17, 164 N. E. 726, 727 (1928) (Cardozo, C. J.) (“Still, as in the days of Edward I, the citizenry may be called upon to enforce the justice of the state, not faintly and with lagging steps, but honestly and bravely and with whatever implements and facilities are convenient and at hand”). See also In re Quarles and Butler, 158 U. S. 532, 535 (1895) (“It is the duty ... of every citizen, to assist in prosecuting, and in securing the punishment of, any breach of the peace of the United States”); Hamilton v. Regents, 293 U. S. 245, 265 n. (1934) (Cardozo, J., concurring) ; Elrod v. Moss, 278 F. 123, 129 (CA4 1921). The concept that citizens have a duty to assist in enforcement of the laws is at least in part the predicate of Fed. Rule Crim. Proc. 17, which clearly contemplates power in the district courts to issue ssubpoenas and subpoenas duces tecum to nonparty witnesses and to hold noncomplying, nonparty witnesses in contempt. Cf. Roviaro v. United States, 353 U. S. 53, 59 (1957) (“The [informer’s] privilege recognizes the obligation of citizens to communicate their knowledge of the commission of crimes to law-enforcement officials and, by preserving their anonymity, encourages them to perform that obligation”). Of course we do not address the question of whether and to what extent such a general duty may be legally enforced in the diverse contexts in which it may arise.
We reject the Court of Appeals’ suggestion that the fact that Congress amended Title III to require that communication common carriers provide necessary assistance in connection with electronic surveillance within the scope of Title III reveals a congressional “doubt that the courts possessed inherent power to issue such orders” and therefore “it seems reasonable to conclude that similar authorization should be required in connection with pen register orders . . . .” 538 F. 2d, at 962. The amendment was passed following the decision of the Ninth Circuit in Application of United States, 427 F. 2d 639 (1970), which held that absent specific statutory authority, a United States District Court was without power to compel a telephone company to assist in a wiretap conducted pursuant to Title III. The court refused to infer such authority in light of Congress’ silence in a statute which constituted a “comprehensive legislative treatment” of wiretapping. Id., at 643. We think that Congress’ prompt action in amending the Act was not an acceptance of the Ninth Circuit’s view but “more in the nature of an overruling of that opinion.” United States v. Illinois Bell Tel. Co., 531 F. 2d, at 813. The meager legislative history of the amendment indicates that Congress was only providing an unequivocal statement of its intent under Title III. See 115 Cong. Rec. 37192 (1969) (remarks of Sen. McClellan). We decline to infer from a congressional grant of authority under these circumstances that such authority was previously lacking. See FTC v. Dean Foods Co,, 384 U. S. 597, 608-612 (1966); Wong Yang Sung v. McGrath, 339 U. S. 33, 47 (1950).
Moreover, even if Congress’ action were viewed as indicating acceptance of the Ninth Circuit’s view that there was no authority for the issuance of orders compelling telephone companies to provide assistance in connection with wiretaps without an explicit statutory provision, it would not follow that explicit congressional authorization was also needed to order telephone companies to assist in the installation and operation of pen registers which, unlike wiretaps, are not regulated by a comprehensive statutory scheme. In any event, by amending Title III Congress has now required that at the Government’s request telephone companies be directed to provide *178assistance in connection with wire interceptions. It is plainly unlikely that Congress intended at the same time to leave federal courts without authority to requiré assistance in connection with pen registers.