The principle underlying the identification of typewriting rests on the improbability of two typewriters possessing, in a number of identical type characters, irregularities of an identical nature. The degree of proof varies in proportion to the nature and number of the irregularities present. Although this type of evidence, in common with all other expert testimony, is confined to the expression of an opinion by the witness, the experience of those who have studied the subject during the past thirty-five years shows that, given favorable conditions, the identification of typewriting can be carried to a state of certainty.