A watermark's a recognizable 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 variations in the paper. There're two main types of watermark, the Dandy Roll process, and the more complex Cylinder Mould process. A watermark's very useful in the examination of paper because it can be used for dating, identifying sizes, mill trademarks and locations, and th… (More on Watermark)