A
watermark is an identifying image or pattern in
paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness or density variations in the paper. Watermarks have been used on
postage stamps,
currency, and other government documents to discourage
counterfeiting. There are two main ways of producing watermarks in paper; the
dandy roll process, and the more complex
cylinder mould process.