<P> A counterfeit banknote detection pen is a pen used to apply an iodine - based ink to banknotes in an attempt to determine their authenticity . </P> <P> Counterfeit banknote detection pens are used to detect counterfeit Swiss franc, euro and United States banknotes, amongst others . Typically, genuine banknotes are printed on paper based on cotton fibers and do not contain the starches that are reactive with iodine . When the pen is used to mark genuine bills, the mark is yellowish or colourless . </P> <P> Such pens are most effective against counterfeit notes printed on standard printer or photocopier paper . The chemical properties of US banknotes prior to 1960 are such that marking pens do not work . </P>

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