Where one hand is more or less passive, the controlling hand doing the writing, its characteristics may be more or less manifest in the writing.  But obviously the controlling hand must be seriously obstructed in its motions by even a passive hand; and since the controlling hand can have no proper or customary rest, the motion must be from the shoulder and with the whole arm.  The writing will therefore be upon an enlarged scale, loose, sprawling, and can have little, if any, characteristic resemblance to the natural and habitual style of the controlling writer, and of course none of the person’s whose hand is passive.

In appearance it changes abruptly from very high or very wide to very low or narrow letters.  This is to be explained by the non-agreement in phase of the impulses due to each of the two writers.  If both are endeavoring at the same moment to write a given stroke the length of that stroke will be measured by the sum of the impulses given by the two writers.  If they act in opposition to one another, one seeking to make a down stroke while the other is trying to make an up stroke, the result will be a line equal to the difference between the stronger and the weaker force.

As these coincidences and oppositions occur at irregular but not infrequent intervals, like the interference and amplification phases of light and sound waves, the result traced on the paper might be expected in advance to be—­and in fact is—­a distorted writing where maxima and minima of effect are connected together by longer or shorter lines of ordinary writing.

The only state of things which can justify the guiding of a hand executing a legal instrument is the feebleness or illness of its owner.

When such assistance is required it is usually given by passing the arm around the body of the invalid and supporting the writing hand while the necessary characters are being made.

Both participants in this action are looking at the writing, and both are thinking of the next letter which must be written, and of the motion of the pen necessary to produce it.  Unless the executing hand were absolutely lifeless or entirely devoid of power, it would be impossible for it not to influence the guiding and presumably stronger hand; for the least force exerted cannot fail to deflect a hand, however strong, in an unnatural and cramped position.  Nor can the hand of the guider fail to add its contribution to the joint effort, however much the brain which controls it may strive to render the hand entirely passive.  Both minds are busy with the same act, and insensibly both hands will write the same letter with the results just described.

Can the characteristics of each hand be separated from those of the other and the relative amount of the two contributions to the joint signature be stated?