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hankronAs long as its been green, US paper currency have been engravings-- a centries old printing process involving the graphics literally carved into metal printing plates. Current and past advanced forgeries have made made with the same engraving process. Highest quality engraving, such as to make a dollar bill, is both technically difficult, time consuming and an art. Rembrandt was an example of a master engraver.
There was once a gang of criminals and forgers, amongst them a master engraver. Whenever this engraver was in jail, the gang had to quit forging money as he was the only one skilled enough to make and use the plates.