JUSTICE BLACKMUN delivered the opinion of the Court.The question presented in this case is whether the exclusion of a defendant from a hearing held to determine the competency of two child witnesses to testify violates the defendant's rights under the Confrontation Clause of the Sixth Amendment or the Due Process Clause of the Fourteenth Amendment.IRespondent Sergio Stincer was indicted in the Circuit Court of Christian County, Ky., and charged with committing first-degree sodomy with T.G., an 8-year-old girl, N.G., a 7-year-old girl, and B.H., a 5-year-old boy, in violation of Ky.Rev.Stat. § 510.070 (1985).  After a jury was sworn, but before the presentation of evidence, the court conducted an in-chambers hearing to determine if the two young girls were competent to testify [Footnote 1]  Over his objection, Page 482 U. S. 733 respondent, but not his counsel (a public defender), was excluded from this hearing.  Tr. 15.The two children were examined separately, and the judge, the prosecutor, and respondent's counsel asked questions of each girl to determine if she were capable of remembering basic facts and of distinguishing between telling the truth and telling a lie.  Id. at 15-26.  T.G., the 8-year-old, was asked her age, her date of birth, the name of her school, the names of her teachers, and the name of her Sunday school.  She was also asked whether she knew what it meant to tell the truth, and whether she could keep a promise to God to tell the truth.  Id. at 16-18. [Footnote 2]  N.G., the 7-year-old girl, was asked similar questions.  Id. at 20-25. [Footnote 3]  The two children were not asked about the substance of the testimony they were to give at trial.  The court ruled that the girls were competent to testify.  Respondent's counsel did not object to these rulings.  Id. at 20, 25.Before each of the girls began her substantive testimony in open court, the prosecutor repeated some of the basic questions regarding the girl's background that had been asked at the competency hearing.  Id. at 31-33 (direct examination of T.G.) (questions regarding age, where the witness attended school and Sunday school, and the like); id. at 66 (direct examination of N.G.) (questions regarding age and where the witness attended school).  T.G. then testified, on direct examination, that respondent had placed a sock over her eyes, had given her chocolate pudding to eat, and then had "put his d-i-c-k" in her mouth.  Id. at 34.  N.G., on direct examination, testified to a similar incident.  Id. at 69. [Footnote 4] Page 482 U. S. 734On cross-examination, respondent's counsel asked each girl questions designed to determine if she could remember past events and if she knew the difference between the truth and a lie.  Some of these questions were similar to those that had been asked at the competency hearing.  See id. at 38-39, 44-47, 60-63 (cross-examination of T.G.); 71-72, 74-75, 78-83 (cross-examination of N.G.).  After the testimony of the girls was concluded, counsel did not request that the trial court reconsider its ruling that the girls were competent to testify. [Footnote 5]  The jury convicted respondent of first-degree sodomy for engaging in deviate sexual intercourse, and fixed his sentence at 20 years' imprisonment. [Footnote 6] Page 482 U. S. 735On appeal to the Supreme Court of Kentucky, respondent argued, among other things, that his exclusion from the competency hearing of the two girls denied him due process and violated his Sixth Amendment right to confront the witnesses against him.  The Kentucky Supreme Court, by a divided vote, agreed that, under the Sixth Amendment of the Federal Constitution and under § 11 of the Bill of Rights of the Kentucky Constitution (the right "to meet the witnesses face to face"), respondent had an absolute right to be present at the competency hearing because the hearing "was a crucial phase of the trial."  712 S.W.2d 939, 940 (1986).  The court explained that respondent's trial "might not have taken place had the trial court determined that the children were not competent to testify."  Id. at 941.  Two justices, however, dissented, concluding that respondent's right to confront the witnesses against him was not violated, because respondent had the opportunity to assist counsel fully in cross-examining the two witnesses at trial.  Id. at 942-944.We granted certiorari, 479 U.S. 1005 (1986), to determine whether respondent's constitutional rights were violated by his exclusion from the competency hearing. [Footnote 7] Page 482 U. S. 736IIAThe Sixth Amendment's Confrontation Clause provides:  "In all criminal prosecutions, the accused shall enjoy the right . . . to be confronted with the witnesses against him."  This right is secured for defendants in state as well as in federal criminal proceedings.  Pointer v. Texas, 380 U. S. 400 (1965).  The Court has emphasized that "a primary interest secured by [the Confrontation Clause] is the right of cross-examination."  Douglas v. Alabama, 380 U. S. 415,  380 U. S. 418 (1965).  The opportunity for cross-examination, protected by the Confrontation Clause, is critical for ensuring the integrity of the factfinding process.  Cross-examination is "the principal means by which the believability of a witness and the truth of his testimony are tested."  Davis v. Alaska, 415 U. S. 308, 316 (1974).  Indeed, the Court has recognized that cross-examination is the "greatest legal engine ever invented for the discovery of truth.'"  California v. Green, 399 U. S. 149,  399 U. S. 158 (1970), quoting 5 J. Wigmore, Evidence § 1367, p. 29 (3d ed.1940).  The usefulness of cross-examination was emphasized by this Court in an early case explicating the Confrontation Clause:"The primary object of the constitutional provision in question was to prevent depositions or ex parte affidavits . . . being used against the prisoner in lieu of a personal examination and cross-examination of the witness in which the accused has an opportunity, not only of testing the recollection and sifting the conscience of the witness, but of compelling him to stand face to face with the jury in order that they may look at him, and judge by his demeanor upon the stand and the manner in which he Page 482 U. S. 737 gives his testimony whether he is worthy of belief."Mattox v. United States, 156 U. S. 237,  156 U. S. 242-243 (1895). [Footnote 8]  See also Kirby v. United States, 174 U. S. 47, 53 (1899).The right to cross-examination, protected by the Confrontation Clause, thus is essentially a "functional" right designed to promote reliability in the truthfinding functions of a criminal trial.  The cases that have arisen under the Confrontation Clause reflect the application of this functional right.  These cases fall into two broad, albeit not exclusive, categories:"cases involving the admission of out-of-court statements and cases involving restrictions imposed by law or by the trial court on the scope of cross-examination."Delaware v. Fensterer, 474 U. S. 15,  474 U. S. 18 (1985) (per curiam).In the first category of cases, the Confrontation Clause is violated when "hearsay evidence [is] admitted as substantive evidence against the defendan[t]," Tennessee v. Street, 471 U. S. 409,  471 U. S. 413 (1985), with no opportunity to cross-examine the hearsay declarant at trial, or when an out-of-court statement of an unavailable witness does not bear adequate indications of trustworthiness.  See Ohio v. Roberts, 448 U. S. 56,  448 U. S. 65-66 (1980).  For example, in Roberts, we held that an out-of-court statement by an unavailable witness was sufficiently reliable to be admitted at trial, consistent with the Confrontation Clause, because defense counsel had engaged in full cross-examination of the witness at the preliminary hearing where the statement was made.  Id. at  448 U. S. 70-73.  In California v. Green, supra, the Court concluded that the Confrontation Clause was not violated by admitting a declarant's inconsistent out-of-court statement "as long as the declarant Page 482 U. S. 738 is testifying as a witness and subject to full and effective cross-examination" at the trial itself.  399 U.S. at  399 U. S. 158.The second category involves cases in which the opportunity for cross-examination has been restricted by law or by a trial court ruling.  In Davis v. Alaska, supra, defense counsel was restricted by state confidentiality provisions from questioning a witness about his juvenile criminal record, although such evidence might have affected the witness' credibility.  The Court held that the Confrontation Clause was violated because the defendant was denied the right "to expose to the jury the facts from which jurors . . . could appropriately draw inferences relating to the reliability of the witness."  415 U.S. at  415 U. S. 318.  Similarly, in Delaware v. Van Arsdall, 475 U. S. 673 (1986), defense counsel was precluded by the trial court from questioning a witness about the State's dismissal of a pending public drunkenness charge against him.  The Court concluded:"By thus cutting off all questioning about an event . . . that a jury might reasonably have found furnished the witness a motive for favoring the prosecution in his testimony,"the trial court's ruling violated the defendant's rights under the Confrontation Clause.  Id. at  475 U. S. 679. [Footnote 9] Page 482 U. S. 739Although claims arising under the Confrontation Clause may not always fall neatly into one of these two categories, these cases reflect the Confrontation Clause's functional purpose in ensuring a defendant an opportunity for cross-examination.  See Lee v. Illinois, 476 U. S. 530 (1986).  Of course, the Confrontation Clause guarantees only"an opportunity for effective cross-examination, not cross-examination that is effective in whatever way, and to whatever extent, the defense might wish."Delaware v. Fensterer, 474 U.S. at  474 U. S. 20 (emphasis in original).  This limitation is consistent with the concept that the right to confrontation is a functional one for the purpose of promoting reliability in a criminal trial.BThe Commonwealth argues that respondent's exclusion from the competency hearing of the two children did not violate the Confrontation Clause because a competency hearing is not "a stage of trial where evidence or witnesses are being presented to the trier of fact."  Brief for Petitioner 22.  Cf. Gannett Co. v. DePasquale, 443 U. S. 368,  443 U. S. 394 (1979) (Burger, C.J., concurring).  Distinguishing between a "trial" and a "pretrial proceeding" is not particularly helpful here, however, because a competency hearing may well be a "stage of trial."  In this case, for instance, the competency hearing was held after the jury was sworn, in the judge's chambers, and in the presence of opposing counsel who asked questions Page 482 U. S. 740 of the witnesses. [Footnote 10]  Moreover, although questions regarding the guilt or innocence of the defendant usually are not asked at a competency hearing, the hearing retains a direct relationship with the trial because it determines whether a key witness will testify.  Further, although the preliminary determination of a witness' competency to testify is made at this hearing, the determination of competency is an ongoing one for the judge to make based on the witness' actual testimony at trial.Instead of attempting to characterize a competency hearing as a trial or pretrial proceeding, it is more useful to consider whether excluding the defendant from the hearing interferes with his opportunity for effective cross-examination.  No such interference occurred when respondent was excluded from the competency hearing of the two young girls in this case.  After the trial court determined that the two children were competent to testify, they appeared and testified in open court.  At that point, the two witnesses were subject to full and complete cross-examination, and were so examined.  Tr. 38-58 (cross-examination of T.G.); id. at 71-84 (cross-examination of N.G.).  Respondent was present throughout this cross-examination, and was available to assist his counsel as necessary.  There was no Kentucky rule of law, nor any ruling by the trial court, that restricted respondent's ability to cross-examine the witnesses at trial.  Any questions asked during the competency hearing, which respondent's counsel attended and in which he participated, could have been repeated during direct examination and cross-examination of the witnesses in respondent's presence.  See California v. Green, 399 U.S. at  399 U. S. 159 ("[T]he inability to cross-examine the witness at the time he made his prior statement cannot easily be shown to be of crucial significance as long as the defendant is assured of full and effective cross-examination at the time of trial"). Page 482 U. S. 741Moreover, the type of questions that were asked at the competency hearing in this case were easy to repeat on cross-examination at trial.  Under Kentucky law, when a child's competency to testify is raised, the judge is required to resolve three basic issues:  whether the child is capable of observing and recollecting facts, whether the child is capable of narrating those facts to a court or jury, and whether the child has a moral sense of the obligation to tell the truth.  See Moore v. Commonwealth, 384 S.W.2d 498, 500 (Ky.1964) ("When the competency of an infant to testify is properly raised, it is then the duty of the trial court to carefully examine the witness to ascertain whether she (or he) is sufficiently intelligent to observe, recollect and narrate the facts and has a moral sense of obligation to speak the truth"); Capps v. Commonwealth, 560 S.W.2d 559, 560 (Ky.1977); Hendricks v. Commonwealth, 550 S.W.2d 551, 554 (Ky.1977); see also Thomas v. Commonwealth, 300 Ky. 480, 481-482, 189 S.W.2d 686, 686-687 (1945); Comment, An Overview of the Competency of Child Testimony, 13 No.Ky.L.Rev. 181, 184 (1986). [Footnote 11]  Thus, questions at a competency hearing usually are limited to matters that are unrelated to the basic issues of the trial.  Children often are asked their names, where they go to school, how old they are, whether they know who the judge is, whether they know what a lie is, and whether they know what happens when one tells a lie.  See Comment, The Competency Requirement for the Child Victim of Sexual Abuse:  Must We Abandon It?, 40 U.Miami L.Rev. 245, 263, and n. 78 (1985); Comment, Defendants' Rights in Child Witness Competency Hearings:  Establishing Page 482 U. S. 742 Constitutional Procedures for Sexual Abuse Cases, 69 Minn.L.Rev. 1377, 1381-1383, and nn. 9-11 (1985). [Footnote 12]In Kentucky, as in certain other States, it is the responsibility of the judge, not the jury, to decide whether a witness is competent to testify based on the witness' answers to such questions.  Whitehead v. Stith, 268 Ky. 703, 709, 105 S.W.2d 834, 837 (1937) (question of competency is one for court, not jury, and if court finds witness lacks qualification, "it commits a palpable abuse of its discretion" should it then permit witness to testify); Payne v. Commonwealth, 623 S.W.2d 867, 878 (Ky.1981); Capps v. Commonwealth, 560 S.W.2d Page 482 U. S. 743 at 560.  See 2 Wigmore § 507, p. 714 (citing cases).  In those States where the judge has the responsibility for determining competency, that responsibility usually continues throughout the trial. [Footnote 13]  A motion by defense counsel that the court reconsider its earlier decision that a child is competent may be raised after the child testifies on direct examination, see, e.g., In re R. R., 79 N.J. 97, 106, 398 A.2d 76, 80 (1979) (at close of State's case, defense attorney moved that 4-year-old boy be declared incompetent on basis of actual testimony given by boy), [Footnote 14] or after direct and cross-examination of the witness.  See, e.g., Reply Brief for Petitioner 12 ("If, during trial, there arises some basis for challenging the judge's competency determination, the judge may be asked to reconsider," referring to respondent's motion to that effect, Tr. 126-127).  Moreover, appellate courts reviewing a trial judge's determination of competency also often will look at the full testimony at trial. [Footnote 15] Page 482 U. S. 744In this case, both T.G. and N.G. were asked several background questions during the competency hearing, as well as several questions directed at what it meant to tell the truth.  Some of the questions regarding the witnesses' backgrounds were repeated by the prosecutor on direct examination, while others -- particularly those regarding the witnesses' ability to tell the difference between truth and falsehood -- were repeated by respondent's counsel on cross-examination.  At the close of the children's testimony, respondent's counsel, had he thought it appropriate, was in a position to move that the court reconsider its competency rulings on the ground that the direct and cross-examination had elicited evidence that the young girls lacked the basic requisites for serving as competent witnesses. [Footnote 16]  Thus, the critical tool of cross-examination was available to counsel as a means of establishing that the witnesses were not competent to testify, as well as a means of undermining the credibility of their testimony.Because respondent had the opportunity for full and effective cross-examination of the two witnesses during trial, and because of the nature of the competency hearing at issue in this case, we conclude that respondent's rights under the Confrontation Clause were not violated by his exclusion from the competency hearing of the two girls. [Footnote 17] Page 482 U. S. 745IIIRespondent argues that his rights under the Due Process Clause of the Fourteenth Amendment were violated by his exclusion from the competency hearing. [Footnote 18]  The Court has assumed that, even in situations where the defendant is not actually confronting witnesses or evidence against him, he has a due process right"to be present in his own person whenever his presence has a relation, reasonably substantial, to the fulness of his opportunity to defend against the charge."Snyder v. Massachusetts, 291 U. S. 97,  291 U. S. 105-106 (1934).  Although the Court has emphasized that this privilege of presence is not guaranteed "when presence would be useless, or the benefit but a shadow," id. at  291 U. S. 106-107, due process clearly requires that a defendant be allowed to be present "to the extent that a fair and just hearing would be thwarted by his absence," id. at  291 U. S. 108.  Thus, a defendant is guaranteed the right to be present at any stage of the criminal proceeding that is critical to its outcome if his presence would contribute to the fairness of the procedure.We conclude that respondent's due process rights were not violated by his exclusion from the competency hearing in this case.  We emphasize, again, the particular nature of the competency hearing.  No question regarding the substantive testimony that the two girls would have given during trial Page 482 U. S. 746 was asked at that hearing.  All the questions, instead, were directed solely to each child's ability to recollect and narrate facts, to her ability to distinguish between truth and falsehood, and to her sense of moral obligation to tell the truth. [Footnote 19]  Thus, although a competency hearing in which a witness is asked to discuss upcoming substantive testimony might bear a substantial relationship to a defendant's opportunity better to defend himself at trial, that kind of inquiry is not before us in this case. [Footnote 20] Page 482 U. S. 747Respondent has given no indication that his presence at the competency hearing in this case would have been useful in ensuring a more reliable determination as to whether the witnesses were competent to testify.  He has presented no evidence that his relationship with the children, or his knowledge of facts regarding their background, could have assisted either his counsel or the judge in asking questions that would have resulted in a more assured determination of competency.  On the record of this case, therefore, we cannot say that respondent's rights under the Due Process Clause of the Fourteenth Amendment were violated by his exclusion from the competency hearing. [Footnote 21]  As was said in United States v. Gagnon, 470 U. S. 522,  470 U. S. 527 (1985) (per curiam), there is no indication that respondent "could have done [anything] had [he] been at the [hearing] nor would [he] have gained anything by attending." [Footnote 22] Page 482 U. S. 748The judgment of the Supreme Court of Kentucky is reversed.It is so ordered
U.S. Supreme CourtKentucky v. Stincer, 482 U.S. 730 (1987)Kentucky v. StincerNo. 86-572Argued April 22, 1987Decided June 19, 1987482 U.S. 730SyllabusAfter a jury was sworn at respondent's Kentucky trial for committing sodomy with two minor girls, but before the presentation of evidence, the court conducted an in-chambers hearing to determine the girls' competency to testify.  Respondent, but not his counsel, was excluded from this hearing.  Under Kentucky law, when a child's competency to testify is raised, the judge is required to resolve whether the child is capable of observing, recollecting, and narrating the facts, and whether the child has a moral sense of the obligation to tell the truth.  Thus, during the hearing, the judge and the attorneys limited themselves to questions designed to determine whether the girls were capable of remembering basic facts and of distinguishing between truth and falsehood.  The judge ruled that both girls were competent to testify.  Before each girl began her substantive testimony in open court, the prosecutor repeated some of the background questions asked at the hearing, while respondent's counsel, on cross-examination, repeated other such questions, particularly those regarding the girls' ability to distinguish truth from lies.  After the girls' testimony was complete, respondent's counsel did not request that the court reconsider its competency rulings.  Respondent was convicted, but the Kentucky Supreme Court reversed, holding that respondent's exclusion from the competency hearing violated his right to confront the witnesses against him.Held:1. Respondent's rights under the Confrontation Clause of the Sixth Amendment were not violated by his exclusion from the competency hearing.  Pp.  482 U. S. 736-744.(a) The Confrontation Clause's functional purpose is to promote reliability in criminal trials by ensuring a defendant an opportunity for cross-examination.  Pp.  482 U. S. 736-739.(b) Rather than attempting to determine whether a competency hearing is a "stage of trial" (as opposed to a pretrial proceeding) subject to the Confrontation Clause's requirements, the more useful inquiry is whether excluding the defendant from the hearing interferes with his opportunity for cross-examination.  No such interference occurred here, because the two girls were cross-examined in open court with respondent present and available to assist his counsel, and because any questions Page 482 U. S. 731 asked during the hearing could have been repeated during direct and cross-examination.  Moreover, the nature of the competency hearing militates against finding a Confrontation Clause violation, because questions at such hearings usually are limited to matters unrelated to basic trial issues.  In addition, the judge's responsibility to determine competency continues throughout the trial, so that a competency determination may be reconsidered on motion after the substantive examination of the child.  Pp.  482 U. S. 739-744.2. Respondent's rights under the Due Process Clause of the Fourteenth Amendment were not violated by his exclusion from the competency hearing.  The defendant's due process right to be present at critical stages of a criminal proceeding if his presence would contribute to the fairness of the procedure is not implicated here in light of the particular nature of the competency hearing, whereby questioning was limited to competency issues and neither girl was asked about the substantive testimony she would give at trial.  There is no indication that respondent's presence at the hearing would have been useful in ensuring a more reliable competency determination.  Pp.  482 U. S. 745-747.712 S.W.2d 939, reversed.BLACKMUN, J., delivered the opinion of the Court, in which REHNQUIST, C.J., and WHITE, POWELL, O'CONNOR, and SCALIA, JJ., joined.  MARSHALL, J., filed a dissenting opinion, in which BRENNAN and STEVENS, JJ., joined, post, p.  482 U. S. 748. Page 482 U. S. 732