The question in this case is the propriety of a writ of mandamus issued by the Court of Appeals for the Seventh Circuit to compel the petitioner, a United States District Judge, to vacate a portion of a pretrial order in a criminal case.

Simmie Horwitz, the defendant in a criminal tax evasion case pending before petitioner in the Northern District of Illinois, filed a motion for a bill of particulars, which contained thirty requests for information. The Government resisted a number of the requests, and over the course of several hearings most of these objections were either withdrawn by the Government or satisfied by an appropriate narrowing of the scope of the bill of particulars by petitioner. Ultimately the dispute centered solely on defendant's request number 25. This request sought certain information concerning any oral statements of the defendant relied upon by the Government to support the charge in the indictment. It asked the names and addresses of the persons to whom such statements were made, the times and places at which they were made, whether the witnesses to the statements were government agents and whether any transcripts or memoranda of the statements had been prepared by the witnesses and given to the Government.1 After considerable discussion with counsel for both sides, petitioner ordered the Government to furnish the information. The United States Attorney declined to comply with the order on the grounds that request number 25 constituted a demand for a list of prosecution witnesses and that petitioner had no power under Rule 7(f) of the Federal Rules of Criminal Procedure to require the Government to produce such a list.

Petitioner indicated his intention to dismiss the indictments against Horwitz because of the Government's refusal to comply with his order for a bill of particulars. Before the order of dismissal was entered, however the Government sought and obtained ex parte from the Seventh Circuit a stay of all proceedings in the case. The Court of Appeals also granted the Government leave to file a petition for a writ of mandamus and issued a rule to show cause why such a writ should not issue to compel petitioner to strike request number 25 from his bill of particulars order. This case was submitted on the briefs, and the Court of Appeals at first denied the writ.2 The Government petitioned for reconsideration, however, and the Court of Appeals, without taking new briefs or hearing oral argument, reversed itself and without opinion issued a writ of mandamus directing petitioner 'to vacate his order directing the Government to answer question 25 in defendant's motion for bill of particulars.'3 We granted certiorari, 386 U.S. 955, 87 S.Ct. 1022, 18 L.Ed.2d 102 (1967), because of the wide implications of the decision below for the orderly administration of criminal justice in the federal courts. We vacate the writ and remand the case to the Court of Appeals for further proceedings.

Both parties have devoted substantial argument in this Court to the propriety of petitioner's order. In our view of the case, however, it is unnecessary to reach this question.4 The peremptory writ of mandamus has traditionally been used in the federal courts only 'to confine an inferior court to a lawful exercise of its prescribed jurisdiction or to compel it to exercise its authority when it is its duty to do so.' Roche v. Evaporated Milk Assn., 319 U.S. 21, 26, 63 S.Ct. 938, 941 (1943). While the courts have never confined themselves to an arbitrary and technical definition of 'jurisdiction,' it is clear that only exceptional circumstances amounting to a judicial 'usurpation of power' will justify the invocation of this extraordinary remedy. De Beers Consol. Mines, Ltd. v. United States, 325 U.S. 212, 217, 65 S.Ct. 1130, 1132, 89 L.Ed. 1566 (1945). Thus the writ has been invoked where unwarranted judicial action threatened 'to embarrass the executive arm of the government in conducting foreign relations,' Ex parte Republic of Peru, 318 U.S. 578, 588, 63 S.Ct. 793, 799, 87 L.Ed. 1014 (1943), where it was the only means of forestalling intrusion by the federal judiciary on a delicate area of federal-state relations, State of Maryland v. Soper, 270 U.S. 9, 46 S.Ct. 185, 70 L.Ed. 449 (1926), where it was necessary to confine a lower court to the terms of an appellate tribunal's mandate, United States v. United States Dist. Court, 334 U.S. 258, 68 S.Ct. 1035, 92 L.Ed. 1351 (1948), and where a district judge displayed a persistent disregard of the Rules of Civil Procedure promulgated by this Court, La Buy v. Howes Leather Co., 352 U.S. 249, 77 S.Ct. 309, 1 L.Ed.2d 290 (1957); see McCullough v. Cosgrave, 309 U.S. 634, 60 S.Ct. 703, 84 L.Ed. 992 (1940); Los Angeles Brush Mfg. Corp. v. James, 272 U.S. 701, 706, 707, 47 S.Ct. 286, 288, 71 L.Ed. 481 (1927) (dictum). And the party seeking mandamus has 'the burden of showing that its right to issuance of the writ is 'clear and indisputable." Bankers Life & Cas. Co. v. Holland, 346 U.S. 379, 384, 74 S.Ct. 145, 148, 98 L.Ed. 106 (1953); see United States ex rel. Bernardin v. Duell, 172 U.S. 576, 582, 19 S.Ct. 286, 287 (1899).

We also approach this case with an awareness of additional considerations which flow from the fact that the underlying proceeding is a criminal prosecution. All our jurisprudence is strongly colored by the notion that appellate review should be postponed, except in certain narrowly defined circumstances, until after final judgment has been rendered by the trial court. See, e.g., Judiciary Act of 1789, §§ 21, 22, 25, 1 Stat. 73, 83, 84, 85; Cobbledick v. United States, 309 U.S. 323, 326, 60 S.Ct. 540, 541, 84 L.Ed. 783 (1940); McLish v. Roff, 141 U.S. 661, 12 S.Ct. 118, 35 L.Ed. 893 (1891). This general policy against piecemeal appeals takes on added weight in criminal cases, where the defendant is entitled to a speedy resolution of the charges against him. DiBella v. United States, 369 U.S. 121, 126, 82 S.Ct. 654, 657, 7 L.Ed.2d 614 (1962). Moreover, 'in the federal jurisprudence, at least, appeals by the Government in criminal cases are something unusual, exceptional, not favored,' Carroll v. United States, 354 U.S. 394, 400, 77 S.Ct. 1332, 1336, 1 L.Ed.2d 1442 (1957), at least in part because they always threaten to offend the policies behind the double-jeopardy prohibition, cf. Fong Foo v. United States, 369 U.S. 141, 82 S.Ct. 671, 7 L.Ed.2d 629 (1962). Government appeal in the federal courts has thus been limited by Congress to narrow categories of orders terminating the prosecution, see 18 U.S.C. § 3731, and the Criminal Appeals Act is strictly construed against the Government's right of appeal, Carroll v. United States, 354 U.S. 394, 399—400, 77 S.Ct. 1332, 1335—1336 (1957). Mandamus, of course, may never be employed as a substitute for appeal in derogation of these clear policies. E.g., Fong Foo v. United States, 369 U.S. 141, 82 S.Ct. 671 (1962); Parr v. United States, 351 U.S. 513, 520—521, 76 S.Ct. 912, 917, 100 L.Ed. 1377 (1956); Bank of Columbia v. Sweeny, 1 Pet. 567, 569, 7 L.Ed. 265 (1828). Nor is the case against permitting the writ to be used as a substitute for interlocutory appeal 'made less compelling * * * by the fact that the Government has no later right to appeal.' DiBella v. United States, 369 U.S. 121, 130, 82 S.Ct. 654, 659 (1962).5 This is not to say that mandamus may never be used to review procedural orders in criminal cases. It has been invoked successfully where the action of the trial court totally deprived the Government of its right to initiate a prosecution. Ex parte United States, 287 U.S. 241, 53 S.Ct. 129, 77 L.Ed. 283 (1932), and where the court overreached its judicial power to deny the Government the rightful fruits of a valid conviction, Ex parte United States, 242 U.S. 27, 37 S.Ct. 72, 61 L.Ed. 129 (1916). But this Court has never approved the use of the writ to review an interlocutory procedural order in a criminal case which did not have the effect of a dismissal. We need not decide under what circumstances, if any, such a use of mandamus would be appropriate. It is enough to note that we approach the decision in this case with an awareness of the constitutional precepts that a man is entitled to a speedy trial and that he may not be placed twice in jeopardy for the same offense.

In light of these considerations and criteria, neither the record before us nor the cryptic order of the Court of Appeals justifies the invocation of the extraordinary writ in this case.

We do not understand the Government to argue that petitioner was in any sense without 'jurisdiction' to order it to file a bill of particulars.6 Suffice it to note that Rule 7(f) of the Federal Rules of Criminal Procedure specifically empowers the trial court to 'direct the filing of a bill of particulars,'7 and that federal trial courts have always had very broad discretion in ruling upon requests for such bills, compare Wong Tai v. United States, 273 U.S. 77, 82 47 S.Ct. 300, 302, 71 L.Ed. 545 (1927). Furthermore, it is not uncommon for the Government to be required to disclose the names of some potential witnesses in a bill of particulars, where this information is necessary or useful in the defendant's preparation for trial. See, e.g., United States v. White, 370 F.2d 559 (7th Cir. 1966). See also United States v. Debrow, 346 U.S. 374, 378, 74 S.Ct. 113, 115, 98 L.Ed. 92 (1953).

The Government seeks instead to justify the employment of the writ in this instance on the ground that petitioner's conduct displays a 'pattern of manifest noncompliance with the rules governing federal criminal trials.'8 It argues that the federal rules place settled limitations upon pretrial discovery in criminal cases, and that a trial court may not, in the absence of compelling justification, order the Government to produce a list of its witnesses in advance of trial. It argues further that in only one category of cases, i.e., prosecutions for treason and other capital offenses, is the Government required to turn over to the defense such a list of its witnesses. A general policy of requiring such disclosure without a particularized showing of need would, it is contended, offend the informant's privilege. Petitioner, according to the Government, adopted 'a uniform rule in his courtroom requiring the government in a criminal case to furnish the defense, on motion for a bill of particulars, a list of potential witnesses.'9 The Government concludes that since petitioner obviously had no power to adopt such a rule, mandamus will lie under this Court's decision in La Buy v. Howes Leather Co., 352 U.S. 249, 77 S.Ct. 309, 1 L.Ed.2d 290 (1957), to correct this studied disregard of the limitations placed upon the district courts by the federal rules.10

The action of the Court of Appeals cannot, on the record before us, bear the weight of this justification. There is absolutely no foundation in this record for the Government's assertions concerning petitioner's practice. The legal proposition that mandamus will lie in appropriate cases to correct willful disobedience of the rules laid down by this Court is not controverted. But the position of the Government rests on two central factual premises: (1) that petitioner in effect ordered it to produce a list of witnesses in advance of trial; and (2) that petitioner took this action pursuant to a deliberately adopted policy in disregard of the rules of criminal procedure. Neither of these premises finds support in the record.

Petitioner repeatedly and, we think, correctly emphasized that request number 25 did not call for a list or government witnesses. He carefully noted that it was utterly immaterial under the terms of request number 25 whether the Government planned to call any of the individuals whose names were sought to the witness stand during the trial. Furthermore, it is clear as a practical matter that the Government's proof in this case, as in any prosecution of this complex nature, will extend far beyond mere damaging admissions of the defendant, and that witnesses will in all probability be called who have never heard Horwitz make any incriminating statements. Nor, if the list of people who have allegedly heard Horwitz make damaging admissions is long, is it likely that they will all be called to testify for the Government. Thus while the two categories have a clear probable overlap, they are not co-extensive. And, as petitioner stated in the opinion accompanying his original order to the Government to file a bill of particulars:

'The reason for requiring disclosure of their names * * * is not that they will or may be witnesses, but that the defendant requires identification of the times, places and persons present in order to prepare his defense.'

Indeed, petitioner excused the Government from answering request number 29(a), which was so broad as to constitute in effect a demand for a list of prosecution witnesses. Finally, it should be noted that in the opinion accompanying the original order, petitioner averred his willingness to narrow the order of disclosure upon a showing by the Government 'that such disclosure will involve physical risk to the individuals or prejudice the government in its ability to produce its evidence.' He repeated this offer numerous times in the subsequent hearings on the Government's objections to the bill, but the United States Attorney never suggested that such a showing could be made in this case.11

The record is equally devoid of support for the notion that petitioner had adopted a deliberate policy in open defiance of the federal rules in matters of pretrial criminal discovery. The extended colloquy between petitioner and government counsel reveals at most that petitioner took a generally liberal view of the discovery rights of criminal defendants.12 But petitioner was careful never to divorce his ruling from his view of the legitimate needs of the defendant in the case before him, and there is no indication that he considered the case to be governed by a uniform and inflexible rule of disclosure.13 Thus the most that can be claimed on this record is that petitioner may have erred in ruling on matters within his jurisdiction. See Parr v. United States, 351 U.S. 513, 520, 76 S.Ct. 912, 917, 100 L.Ed. 1377 (1956). But '(t)he extraordinary writs do not reach to such cases; they may not be used to thwart the congressional policy against piecemeal appeals.' Id., at 520—521, 76 S.Ct., at 917. Mandamus, it must be remembered, does not 'run the gauntlet of reversible errors.' Bankers Life & Cas. Co. v. Holland, 346 U.S. 379, 382, 74 S.Ct. 145, 147 (1953). Its office is not to 'control the decision of the trial court,' but rather merely to confine the lower court to the sphere of its discretionary power. Id., at 383, 74 S.Ct. 148. Thus the record before us simply fails to demonstrate the necessity for the drastic remedy employed by the Court of Appeals.

Even more important in our view, however, than these deficiencies in the record is the failure of the Court of Appeals to attempt to supply any reasoned justification of its action. Had the Government in fact shown that petitioner adopted a policy in deliberate disregard of the criminal discovery rules and that this policy had proved seriously disruptive of the efficient administration of criminal justice in the Northern District of Illinois, it would have raised serious questions under this Court's decision in La Buy v. Howes Leather Co., 352 U.S. 249, 77 S.Ct. 309 (1957).14 In La Buy, however, we specifically relied upon evidence in the record which showed a pattern of improper references of cases to special masters by the District Judge. 352 U.S., at 258, 77 S.Ct., at 314. There is no evidence in this record concerning petitioner's practice in other cases, aside from his own remark that the Government was generally dissatisfied with it,15 and his statements do not reveal any intent to evade or disregard the rules. We do not know what he ordered the Government to reveal under what circumstances in other cases. This state of the record renders the silence of the Court of Appeals all the more critical. We recognized in La Buy that the familiarity of a court of appeals with the practice of the individual district courts within its circuit was relevant to an assessment of the need for mandamus as a corrective measure. See 352 U.S., at 258, 77 S.Ct., at 314. But without an opinion from the Court of Appeals we do not know what role, if any, this factor played in the decision below. In fact, we are in the dark with respect to the position of the Court of Appeals on all the issues crucial to an informed exercise of our power of review. We do not know: (1) what the Court of Appeals found petitioner to have done; (2) what it objected to in petitioner's course of conduct—whether it was the order in this particular case or some general practice adopted by petitioner in this and other cases;16 (3) what it thought was the proper scope of a bill of particulars under Rule 7(f) and what limitations it thought the criminal rules placed upon the particular or generalized discretion of a district court to order the Government to file such a bill; or (4) what relevance, if any, it attached to the fact that this order was entered in a criminal case, in assessing the availability of mandamus. We cannot properly identify the questions for decision in the case before us without illumination of this unclear record by the measured and exposed reflection of the Court of Appeals.

Due regard, not merely for the reviewing functions of this Court, but for the 'drastic and extraordinary' nature of the mandamus remedy, Ex parte Fahey, 332 U.S. 258, 259, 67 S.Ct. 1558, 1559, 91 L.Ed. 2041 (1947), and for the extremely awkward position in which it places the District Judge, id., at 260, 67 S.Ct., at 1559 demands that a court issuing the writ give a reasoned exposition of the basis for its action.

Mandamus is not a punitive remedy. The entire thrust of the Government's justification for mandamus in this case, moreover, is that the writ serves a vital corrective and didactic function. While these aims lay at the core of this Court's decisions in La Buy and Schlagenhauf v. Holder, 379 U.S. 104, 85 S.Ct. 234 (1964), we fail to see how they can be served here without findings of fact by the issuing court and some statement of the court's legal reasoning. A mandamus from the blue without rationale is tantamount to an abdication of the very expository and supervisory functions of an appellate court upon which the Government rests its attempt to justify the action below.

The preemptory common-law writs are among the most potent weapons in the judicial arsenal. 'As extraordinary remedies, they are reserved for really extraordinary causes.' Ex parte Fahey, 332 U.S. 258, 260, 67 S.Ct. 1558, 1559 (1947). There is nothing in the record here to demonstrate that this case falls into that category, and thus the judgment below cannot stand. What might be the proper decision upon a more complete record, supplemented by the findings and conclusions of the Court of Appeals, we cannot and do not say. Hence the writ is vacated and the cause is remanded to the Court of Appeals for the Seventh Circuit for further proceedings not inconsistent with this opinion. It is so ordered.

Writ vacated and cause remanded.

Mr. Justice MARSHALL took no part in the consideration or decision of this case.

Mr. Justice BLACK, concurring.

I concur in the Court's judgment to vacate and agree substantially with its opinion, but would like to add a few words, which I do not understand to be in conflict with what the Court says, concerning the writ of mandamus. I agree that mandamus is an extraordinary remedy which should not be issued except in extraordinary circumstances. And I also realize that sometimes the granting of mandamus may bring about the review of a case as would an appeal. Yet this does not deprive a court of its power to issue the writ. Where there are extraordinary circumstances, mandamus may be used to review an interlocutory order which is by no means 'final' and thus appealable under federal statutes. Finality, then, while relevant to the right of appeal, is not determinative of the question when to issue mandamus. Rather than hinging on this abstruse and infinitely uncertain term, the issuance of the writ of mandamus is proper where a court finds exceptional circumstances to support such an order. In the present case it is conceivable that there are valid reasons why the Government should not be forced to turn over the requested names and that compliance with the order would inflict irreparable damage on its conduct of the case. The trouble here, as I see it, is that neither of the courts below gave proper consideration to the possible existence of exceptional facts which might justify the Government's refusal to disclose the names. Having no doubt as to the appropriateness of mandamus, if the circumstances exist to justify it, I would vacate the judgment below and remand the case to the Court of Appeals for further deliberation on whether there are special circumstances calling for the issuance of mandamus.