To the untrained eye, many markings on today's currency look arbitrary, and indeed many marks may only be significant to historians. But other markings serve a purpose whose significance is obvious to many: to safeguard the value of currency by making it easy to tell real banknotes from counterfeit ones.
Banknotes of countries across the world have been styled certain ways to reflect the character and the identity of the countries that issued them, and so they feature stylized depictions of famous heroes and various national symbols. However, many times in the past, countries have been forced to take certain measures to ensure that the authentic banknotes they issued could not be confused with fake banknotes issued by counterfeiters. This is why modern currencies the world over incorporate both historically significant features and cutting-edge security features.
A hologram is a photographic image (mostly of a famous person or a building) placed on a banknote using a special process that results in it appearing to have depth and to be a three-dimensional image. The process that goes into creating this image is complex, making duplication of the hologram exceedingly difficult. As a result, it has become a standard feature of banknotes and credit cards for security purposes and for warding off identity theft and counterfeiters.
A watermark is a second security feature of currency. It is a pattern in the paper that can appear as shadows or an image of varying visibility; it may not be immediately apparent to the naked eye. In some cases, watermarks require special treatment to be made visible, such as ultraviolet lighting or the use of a special fluid that dampens the paper but does not damage it.
Threads are a third security feature of currency. Special threads that may not be immediately visible, or that may be invisible to the naked eye, are incorporated into the paper to protect against counterfeiters. These threads may be fluorescent, metallic and/or printed with tiny images or text to make them even harder to counterfeit. In addition, threads can be specially treated so that they show up only on one side of the banknote.
As mentioned previously, not all the marks on currency are placed there for security purposes. The choice of marks for a country's currency may be based on such considerations as immediate ease of use (i.e., numbers to denote the value or denomination of the bill); legalization (i.e., the signatures of the requisite officials, to make the bill legal tender); identification (i.e., the official flag, seal and other such insignia of the country); and various items of historical significance.