In the United States a witness may be asked to write on cross-examination, but not in direct.

Before a paper can be accepted as a standard of comparison it must be proved to be genuine to the satisfaction of the judge.  His decision on this question is final if supported by proper evidence.  In some states the question of genuineness is for the jury.

A party denying his handwriting may be asked on cross-examination, if his signature to another instrument is genuine.  This is the test which may be successfully applied to ascertain if the signature is genuine.  A plaintiff, on one occasion, denied most positively that a receipt produced was in his handwriting.  It was thus worded, “Received the Hole of the above.”  On being asked to write a sentence in which the word “whole” was introduced, he took evident pains to disguise his handwriting, but he adopted the phonetic style of spelling, and also persisted in using the capital H.

The practice of thus testing a witness is vindicated by one of the most sagacious of German jurists, Mittermaier, on grounds not only of expediency, but of authority.

Comparison of handwriting, either by jury or witness, is uniformly allowed to prove writings which are not old enough to prove themselves, but are too old to admit of direct proof of their genuineness.

Handwriting, considered under the law of evidence, includes not only the ordinary writing of one able to write, but also writing done in a disguised hand, or in cipher, and a mark made by one able or unable to write.

The principles regulating the proof of handwriting apply equally to civil and criminal cases.

The paper the handwriting of which is sought to be proved by experts must ordinarily be produced in court, but such production will be excused when the paper has been lost or destroyed and when it is a public record, which cannot be brought into court.

Genuineness may be proved in all cases, except where paper is required to be identified by an official seal, and except as controlled by law applicable to attested instruments.

It may be proved by his own admissions; by witnesses who saw the party write; by witnesses who corresponded with the party; by witnesses who had seen papers acknowledged by the party; by witnesses having personal relations with the party.

Comparison of handwriting, technically called presumptio ex scripto nunv viso, is where a paper or papers are proved or admitted to be in a party’s handwriting, and a witness entirely unacquainted with the party’s handwriting, or the jury, is allowed to make a comparison by juxtaposition of the writing so proved or admitted, and the writing disputed.

All evidence of handwriting, except where the witness sees the documents written, is in its nature comparison.  It is the belief which a witness entertains upon comparing the writing in question with an exemplar in his mind derived from some previous writing.