A watermark is a recognizable image or pattern in paper or plastic cards that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), created by thickness and density variations in the paper or plastic cards. There are two main types of watermark, the Fourdrinier or Dandy Roll process where the watermark is pressed into the paper and thus provides less definition but higher paper strength and the Cylinder Mold process where the paper fibers are formed around a watermark mold, providing higher image definition but lower paper strength.