Engraving
Because of the high level of detail that can be done by a master engraver, faking engraved designs is almost impossible, and modern banknotes are almost always engraved, as are plates for printing money, checks, bonds and other papers that should not be faked. Engraving is so fine that a normal printer can not make the detail of hand engraved images properly. In the U.S. Bureau of Engraving and Printing, more than one engraver will work on the same printing plate, making it nearly impossible for any person to duplicate all the engraving on almost any banknote or document.