forgery, in criminal law, willful fabrication or alteration of a written document with the intent to injure the interests of another in a fraudulent manner. The crime may be committed even though the fraudulent scheme fails. The forgery of government obligations—e.g., money, bonds, postage stamps—constitutes the separate offense of counterfeiting. Typical examples of forgery are making insertions or alterations in otherwise valid documents and appending another's signature to a document without permission. It is, of course, lawful to sign another's signature as his attorney or representative so long as there is no plan to commit fraud. Most instances of forgery occur in connection with instruments for the payment of money. The crime may also concern documents of title, e.g., deeds, or public documents, including birth and marriage certificates. In the United States forgery ordinarily is a state crime; but to send forged documents through the post office may constitute the federal crime of mail fraud.