Most countries have banknotes with watermarks.  The USA added a watermark
to its new $100 note in 1996 (and some people in the USA were curious if it
was some kind of subliminal message).

  A watermark is simply a design that's within the paper of the note itself
which can only be seen when viewing the note with a strong light behind it
(i.e. you can only see it when light is passing through the note).  Watermarks
are an effective, although very old, anti-counterfeiting device.



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3. USA PAPER MONEY

Some parts © 2009 Advameg, Inc.