One feature on most banknotes is a watermark. The watermark is an image that is formed onto the banknote paper before the printing process. The image is made using a fine screen mesh that has a super thin impression of the image on it. The paper is then subjected to the screen mesh and the result is that the paper retains the image in black and white with shades of gray. The watermark was invented in Italy in the late 13th century and the first images were of crosses and Fleur d' lis. Watermarks have been used on banknotes since 1697 when the Bank of England incorporated them into their papers.
To view a watermark, hold the paper to a light source and the image will be visible as the light transmits through the paper, but some have a heavier impression and they are readily seen on the banknote. Most modern notes have a specific area for the watermark, and it is usually in a non printed area of the note in a circle or oval shape. The images range from portraits of the person on the banknote to letters, symbols, animals and just about anything else. Earlier notes have had the whole banknote paper covered with a watermark design. The watermark is the oldest banknote counterfeit deterrent. Their effectiveness today is that they cannot be adequately photographed or copied onto paper by counterfeiters.
Below are some watermarks from my personal banknote collection. You can see that there are a variety of styles that are used in these banknotes from eight different countries. Amazingly, the watermark is often overlooked as a prominent feature of banknotes because they aren't as visible to the eye, but as you can see, they can be very detailed and quite beautiful.