At a trial, the origin and authenticity of important documents may be proved by experts known as "questioned document examiners." These examiners apply scientific techniques to explain when and how a particular document came to be, and whether a particular signature or other handwriting on the document is authentic or a forgery. Judges routinely admit testimony such as the following into evidence:

• Questioned document examiners may compare two handwriting samples and give an opinion as to whether the same person wrote both. For example, the testimony of questioned document examiners who looked at ransom notes was crucial to the 1932 conviction of Bruno Hauptmann for kidnapping and killing the Lindbergh baby. The experts testified that, among many other things, both Hauptmann and the author of the ransom notes wrote "t" in the word "the" in such a way that it resembled a "u."

• Experts may give an opinion as to the make and model typewriter used to prepare a typed document. For example, an expert may trace vertical misalignment in certain characters to a particular typewriter. This same type of expertise is now being applied to computer generated documents and the printers that produce them.

• Experts often restore burned or water-damaged documents sufficiently to read them.