Big purple digit

I got a five dollar bill in change while shopping today.  To my
surprise, there was a big purple "5" in one corner.

I'm curious what the point in constantly changing the appearance of US
currency is.  I can't see how it can discourage counterfeiting, since
the old bills are still in circulation and are still legal tender.  If
the old bills were easier to forge, nothing keeps counterfeiters from
continuing to produce them.

If nobody knows what new bills look like, counterfeiters can produce
anything remotely plausible, and who would be able to tell it wasn't
genuine?
-- 
Keith F. Lynch - http://keithlynch.net/
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