<Li> Other features for machine authentication and processing of the currency . </Li> <P> Annual releases of the 1996 series followed . The $50 note June 12, 1997, introduced a large dark numeral with a light background on the back of the note to make it easier for people to identify the denomination . The $20 note in 1998 introduced a new machine - readable capability to assist scanning devices . The security thread glows green under ultraviolet light, and "USA TWENTY" and a flag are printed on the thread, while the numeral "20" is printed within the star field of the flag . The microprinting is in the lower left ornamentation of the portrait and in the lower left corner of the note front . As of 1998, the $20 note was the most frequently counterfeited note in the United States . The new design of the $5 and $10 notes were released in 2000 . </P> <P> May 13, 2003, The Treasury announced that it would introduce new colors into the $20 bill, the first U.S. currency since 1905 (not counting the 1934 gold certificates) to have colors other than green or black . The move was intended primarily to reduce counterfeiting, rather than to increase visual differentiation between denominations . The main colors of all denominations, including the new $20 and $50, remain green and black; the other colors are present only in subtle shades in secondary design elements . This contrasts with notes of the euro, Australian dollar, and most other currencies, where strong colours are used to distinguish each denomination from the other . </P> <P> The new $20 bills entered circulation October 9, 2003, the new $50 bills, September 28, 2004 . The new $10 notes were introduced in 2006 and redesigned $5 bills began to circulate March 13, 2008 . Each will have subtle elements of different colors, though will continue to be primarily green and black . The Treasury said it will update Federal Reserve notes every 7 to 10 years to keep up with counterfeiting technology . In addition, there have been rumors that future banknotes will use embedded RFID microchips as another anti-counterfeiting tool . </P>

When did paper money start in the us