The requirement of Amendment VI that the accused in a criminal prosecution must be confronted with the witnesses against him is simply a statement of a rule of common-law procedure in prosecutions for crime which is generally recognized in the constitutions of the states. The purpose of such requirement is to enable the accused to subject the witnesses against him to the tests of credibility afforded by cross-examination and impeaching evidence, and to have the jury pass upon the weight of their testimony in view of such tests and their general conduct and appearance while testifying. Experience has shown that these are valuable means for arriving at the truth. In civil cases testimony may be authorized to be taken by deposition to be read in evidence without the presence of the witness, the deposition having been given and sworn to before some officer authorized to administer oaths, but the constitutional provision that the accused in a criminal prosecution must be confronted by the witnesses against him prevents the testimony of witnesses for the prosecution being introduced by depositions. The accused may, if he sees fit, waive the constitutional requirement and permit testimony to be given by deposition, and he may introduce such testimony on his own behalf, if authorized by statutory provisions. It may result from this constitutional requirement that the prosecution will be unable to convict, if some essential fact in connection with the commission of the crime can only be proven by witnesses who are outside of the state, as the state cannot compel the attendance of witnesses from beyond its limits.

There are at least two apparent exceptions to the rule requiring the defendant to be confronted by the witnesses against him, which are made by the courts in the practical administration of justice. (1) In a prosecution for criminal homicide the dying declarations of the person killed with reference to the circumstances of the homicide and the connection of the accused therewith may be shown. The reason usually given for this exception is that one who believes himself to be about to die is as likely to tell the truth as though he were under oath and subjected to cross-examination, and the exception as to dying declarations is accordingly limited to statements made by the injured person under the sense of impending death which in fact follows soon after the statements are made. (2) Another apparent exception is found in the admission on a second trial of the testimony given against the accused on a former trial by a witness subsequently deceased; that is, if the accused on one trial is confronted by a witness duly sworn and properly cross-examined, and by reason of the failure of the jury to agree or in case a verdict of guilty has been set aside and a new trial granted, the accused has again been put on trial under the same indictment and the witness testifying on the former trial is dead (or perhaps if he has gone beyond the reach of a subpoena) those who heard his evidence on the former trial may testify what it was and thus make it available against the accused. In such case the accused has in fact once been confronted with the witness who has been required to testify under oath and been subjected to a cross-examination, and there is no good reason why the prosecution should be deprived of the benefit of such testimony by the accident of death or by other casualty not due to any fault or negligence on its part (Mattox v. United States).