The 12 original Dri Mark Counterfeit Money Detector Pen 351R is a set of pens specifically designed for detecting counterfeit currency, such as fake dollar bills. These pens are made of plastic and use a special ink that reacts to the unique features of real currency, making it easy to identify genuine bills. The Dri Mark brand is known for its reliability in detecting counterfeit money, making this set of pens a valuable tool for businesses and individuals looking to protect themselves from fraudulent currency.