design produced by creating a variation in the thickness of paper fibre during the wet-paper phase of papermaking. This design is clearly visible when the paper is held up to a light source.

Watermarks are known to have existed in Italy before the end of the 13th century. Two types of watermark have been produced. The more common type, which produces a translucent design when held up to a light, is produced by a wire design laid over and sewn onto the sheet mold wire (for handmade paper) or attached to the “dandy roll” (for machine-made paper). The rarer “shaded” watermark is produced by a depression in the sheet mold wire, which results in a greater density of fibres—hence, a shaded, or darker, design when held up to a light. Watermarks are often used commercially to identify the manufacturer or the grade of paper. They have also been used to detect and prevent counterfeiting and forgery.

The notion of watermarks as a means of identification was carried beyond the printing press into the computer age. Digital watermarks, which may or may not be visible, can be added to image and video files so that information embedded in the file is retrievable for purposes of copyright protection. Audio files can also be watermarked in this manner.