A watermark is a recognizable image or pattern in paper that shows in various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness variations in the paper. There are two main types of watermark, the Dandy Roll process, and the more complex Cylinder Mould process. A watermark is very useful in the examination of paper because it can be used for dating, identifying sizes, mill trademarks and locations, and the quality of a paper.
Watermarks vary greatly in their visibility; while some are obvious on casual inspection, others require some study to pick out. Various aids have been developed, such as watermark fluid that wets the paper without damaging it. Encoding an identifying code into digitized music, video, picture, or other file is known as a digital watermark.