The principle on which experts claim to be able to detect variations and to differentiate between handwritings is based on the well-established axiom that there is no such thing as a perfect pair in nature; that, however close the apparent similarity between two things, a careful examination and comparison will reveal marked differences to those trained to detect them.

This is especially true of everything that is produced by human agency. Everyone knows how difficult it is to keep check upon and eradicate certain physical habits, such as gestures, style of walking, moving the hands, arms, &c., tricks of speech, or tone of voice. These mannerisms, being mechanical and automatic, or the result of long habit, are performed unconsciously, and there is probably no person who is entirely free from some marked peculiarity of manner, which he is ignorant of possessing. It is a well-known fact that the subject of caricature or mimicry rarely admits the accuracy or justness of the imitation, although the peculiarities so emphasised are plainly apparent to others. Even actors, who are supposed to make a careful study of their every tone and gesture, are constantly criticised for faults or mannerisms plain to the observer, but undetected by themselves.

It is easy, therefore, to understand how a trick or a gesture may become a fixed and unconscious habit through long custom, especially when, as in the case of a peculiarity of style in handwriting, there has been neither criticism on it, nor special reason for abandoning it.

Every person whose handwriting is developed and permanently formed has adopted certain more or less distinctive peculiarities in the formation of letters of which he is generally unaware.

The act of writing is much less a matter of control than may be supposed. The pen follows the thoughts mechanically, and few ready and habitual writers could, if suddenly called upon to do so, say what peculiarities their writing possessed. For example, how many could say off-hand how they dotted an i—whether with a round dot, a tick or a dash—whether the tick was vertical, horizontal or sloping; what was the proportional distance of the dot from the top of the i. Again, ask a practised writer how he crosses the letter t—whether with a horizontal, up or down stroke? It is safe to assume that not one in a thousand could give an accurate answer, for the reason that the dotting of an i and crossing of a t have become mechanical acts, done without thought or premeditation, but as the result of a long-formed habit.

It is these unconscious hand-gestures and mechanical tricks of style that the handwriting expert learns to distinguish and recognise,—the unconsidered trifles that the writer has probably never devoted a minute's thought to, and which come upon him as a surprise when they are pointed out to him. Their detection is rendered the more easy when one knows what to look for from the fact that they are, unlike gestures and tricks of voice, permanent. A mannerism may not strike two observers in the same way, nor is it easy to compare, for it is fleeting, and the memory has to be relied upon to recall a former gesture in order to compare it with the last. It is not so with a hand-gesture in writing. The sign remains side by side with its repetition, for careful and deliberate comparison; and if the writing be a long one, the expert has the advantage of being in possession of ample material on which to base his judgment.

A Popular Fallacy.—One of the most frequent objections offered by the casual critic when the subject of expert testimony is discussed is to the effect that people write different hands with different pens, and he probably believes this to be true. A very slight acquaintance with the principles on which the expert works would satisfy this spontaneous critic of the fallacy of his objection. A person who habitually writes a fine, small hand, sloping from right to left, may believe that he has altered the character of his hand by using a thick, soft quill, reversing the slope to what is called a backhand, and doubling the size of the letters. All he has done is to put on a different suit of clothes; the same man is in them. The use of a thick pen does not make him put a dot over the i where before he made an horizontal dash; it does not turn a straight, barred t into a curved loop, neither does it alter the proportionate distance between the letters and lines. It does not make him form loops where before he habitually made bars, or vice versÂ, and if he formerly made a u with an angle like a v he will not write the u with a rounded hook. Neither will it cause him to drop his habit of adding a spur to his initial letters or curtail the ends and tails that he was wont to make long. In short, the points to which the expert devotes his investigation are those least affected by any variation in the character of the pen used and the hand-gestures which have, by constant usage, become as much part of the writer's style as his walk and the tone of his voice.

It follows, therefore, that the work of the handwriting experts consists in learning how to detect and recognize those unconscious or mechanical signs, characteristics or hand-gestures that are a feature in the handwriting of every person, no matter how closely any two hands may approximate in general appearance. However similar two hands may seem to the casual and untrained observer, very distinct and unmistakable differences become apparent when the student has been taught what to look for. There is no more certain thing than the fact that there has not yet been discovered two handwritings by separate persons so closely allied that a difference cannot be detected by the trained observer. Every schoolmaster knows that in a class of pupils taught writing from the same model, and kept strictly to it, no two hands are alike, although in the early and rudimentary stage, before the hand has attained freedom and approached a settled character, the differences are less marked. So soon as the child has been freed from the restraint of the set copy and the criticism of the teacher, he begins to manifest distinct characteristics, which become more marked and fixed with practice and usage.

There is no writing so uniform as the regulation hand used, and wisely insisted upon, in the Civil Service, and familiar to the general public in telegrams and official letters. Yet it is safe to say that there is not a telegraph or post office clerk in England who would not be able to pick out the writing of any colleague with which he was at all acquainted.

Duplicates non-existent.—But the best and most decisive answer to the objection that writings may be exactly similar lies in the notorious fact that during half a century experts have failed to discover two complete writings by different hands, so much alike that a difference could not be detected. Had such existed, they would long ere this have been produced for the confuting of the expert in the witness-box; particularly when we bear in mind that the liberty, and even the life of a person, have depended upon the identification of handwriting. That there are many cases of extraordinary similarity between different handwritings is a fact; if there were not, there would be very little occasion for the services of the expert, but it is equally a fact that the fancied resemblance becomes less apparent as soon as the writing is examined by a capable and painstaking expert. It should not be forgotten that it is not every person who undertakes the comparison of handwritings who is qualified for the task, any more than every doctor who diagnoses a case can be depended upon to arrive at an accurate conclusion. But if the tried and accepted principles of the art be acted upon, there should be no possibility of error, always assuming that the person undertaking the examination has a sufficiency of material for comparison. An expert who valued his reputation would, for example, be very cautious about giving an emphatic opinion if the only material at his disposal were two or three words or letters. It is quite possible that a clever mimic might reproduce the voice of another person so accurately as to deceive those who knew the subject of the imitation; but let him carry on a conversation in the assumed voice for a few minutes, and detection is certain. In like manner, while a few characters and tricks of style in writing may be fairly well imitated, it is impossible to carry the deception over a number of words. Sooner or later the forger lapses into some trick of his own, and it is here the trained observer catches him. The expert, like the caricaturist, lays himself out to note the peculiarities of his subject, knowing that these are practically beyond the control of the writer, and that the probabilities are that he is not even aware of them. Peculiarities in handwriting, like unchecked habits in children, become, in time, crystallised into a mannerism so fixed as to be part of the nature, and consequently are difficult of eradication. As a matter of fact a peculiarity in handwriting is more often cultivated than controlled, many writers regarding a departure from orthodox copybook form as an evidence of an "educated hand."

The Law of Probabilities.—In examining a writing for comparison with another the expert notes all peculiarities, which he labels, for distinctive reference, "tricks." When he has recorded as many as possible he looks for them in the writing which he has to compare. Suppose that he has taken note of a dozen tricks, and finds them all repeated in the suspected writing. The law of probabilities points to a common authorship for both writings, for it is asking too much to expect one to believe that there should exist two different persons, probably strangers, who possess precisely the same peculiarities in penmanship.

This principle of the law of probabilities is applied in the case of the identification of persons "wanted" by the police. For example, the official description of an absconding forger runs as follows:—"He has a habit of rubbing his right thumb against the middle finger as if turning a ring. He frequently strokes his right eyebrow with right forefinger when engaged in writing; when perplexed, he bites his lower lip and clenches and unclenches his fingers."

Now there are, probably, thousands of people who do every one of these things singly, but the chances are millions to one against there being two people who do them all as described in the official placard. In like manner there may be a multitude of writers who form an f or k with a peculiar exaggerated buckle. Thousands more may make certain letters in the same way, but to assume that there are two persons who possess equally the whole twelve characteristics noted by the expert is to strain coincidence to the breaking-point of absurdity.

Therefore, it follows that it is the weight of cumulative evidence of similarity in the production of unusual tricks of style that proclaims a common authorship for two apparently different writings.

It may be, and often is, the case that the peculiarities or tricks in the original have been imitated in the suspected writing. As the result of his experience in knowing what to look for in a copied document, the expert is not deceived. However good the copy, there are always apparent to the trained eye evidences that prove another and stranger hand, plain as the difference between the firm, clear line of the drawing master and the broken saw-edged effort of the pupil. Habitual observation trains the eye to an extent that would scarcely be credited unless proved by experiment. The art of observation cannot be taught; it must be the outcome of practice. The most the teacher can do is to indicate the lines on which the study should be carried out, and offer hints and suggestions as to what to look for. The rest is in the hands of the student.