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_ ha