Watermarking

A banknote or bank notealso called a bill (North American English) or simply a noteis a type of paper money that is made and distributed ("issued") by a bank of issue, payable to the bearer on demand. Banknotes were originally issued by commercial banks, which were legally required to redeem the notes for legal tender (usually gold or silver coin) when presented to the chief cashier of the originating bank. These commercial banknotes were only traded at face value in the market served by the issuing bank. Commercial banknotes have primarily been replaced by national banknotes issued by central

thumb|Machine-made watermark on a 19th century letter A watermark is an identifying image or pattern in paper that appears as varying shades of light and dark when viewed by either transmitted light or reflected light. These patterns are created from variations in the thickness or density of the paper. Watermarks have historically been used on postage stamps, currency, and other official documents to discourage counterfeiting. There are two primary methods of producing watermarks in paper: the dandy roll process and the more complex cylinder mould process.

marker covertly embedded in a signal

pattern of symbols incorporated into a number of banknote designs

digital watermark which certain color laser printers and copiers leave on every single printed page, allowing to identify the device with which a document was printed and giving clues to the originator

Filigranology is the study of watermarks. It is usually pursued in order to discover information about the date and origin for a paper-based piece of writing or a piece of art. There are several catalogues of watermarks – most notably C. M. Briquet's, Les Filigranes (1907), which dealt mainly with the period up to 1600, and Edward Heawood's Watermarks, Mainly of the 17th and 18th Centuries (1950), both of which include illustrations of many watermarks from dated documents. These catalogues form an important resource for researchers.

method for exposing an information leak by giving different versions of a sensitive document to each of several suspects

Swiss historian (1839-1918)