¶1In the trial of an indictment for an assault with, intent to commit rape upon Catherine M. Stafford, a child about five years and ten months old at the time of the alleged assault, and “just past” six years at the time of the trial, the child was permitted to give material evidence in behalf of the Comm on - wealth. The sole exception is to her competency as a witness.
¶2It is clear that the tender years of the child did not disqualify her. While age is of importance, it is not the test. The child might have been much older and still have been unqualified to give testimony. Her capacity to observe, remember, and give expression to that which she had seen, heard, or experienced, was the crucial consideration.
¶3Much which cannot be reproduced by the printed word depends on the child’s appearance and manner. It is seldom that the discretion of the trial judge can be revised; its exercise must have been clearly erroneous to justify such action. This is because the question for decision is almost always one of fact and hence not reviewable. Commonwealth v. Hutchinson, 10 Mass. 225. Commonwealth v. Robinson, 165 Mass. 426. Commonwealth v. Reagan, 175 Mass. 335. Commonwealth v. Ramage, 177 Mass. 349. Commonwealth v. Marshall, 211 Mass. 86. Commonwealth v. Teregno, 234 Mass. 56. Wheeler v. United States, 159 U. S. 523.
¶4The defendant urges that the witness did not understand the nature of the required oath and for that reason was improperly permitted to testify. As to this no precise line can be defined, and cases placed on one side or the other. The ultimate test cannot be the amount of moral training and religious understanding, but must depend upon the existence of understanding sufficient to comprehend the difference between truth and falsehood, the wickedness of the latter and the obligation and duty to tell the truth, and, in a general way, belief that failure to perform the obligation will result in punishment. The child need not and *326probably will not understand this in all its fulness; it is unnecessary for her to do so. See Wigmore on Ev. §§ 1817, 1821; St. 1870, c. 393, § 1; G. L. c. 233, §§ 15, 19, 20. The question now considered also must be decided by the trial judge in the exercise of a wise discretion, and an appellate tribunal ordinarily will not review his determination. Commonwealth v. Mullins, 2 Allen, 295. O’Connor v. Hallinan, 103 Mass. 547.
¶5If a child does not have the necessary understanding to comprehend the mature of the obligation imposed by the oath of a witness, he may be instructed in open court, or his testimony deferred until such instruction has been given. Commonwealth v. Lynes, 142 Mass. 577.
¶6Applying these long settled principles to this case, no error appears. The child was examined twice, on different days, in the absence of the jury, and instructed by a priest in the interval. After most careful consideration, she was permitted to testify. While the bill of exceptions refers to the rulings of the judge it is evident that no ruling of law was made except such as followed the findings of competency necessarily made by the judge before the witness was permitted to testify.
¶7 Exceptions overruled.