نقش‌آب

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.