U.S. paper currency is difficult to duplicate.
The U.S. Secret Service was established in 1865 to combat rampant counterfeiting. Though reprographic technology has become more sophisticated, counterfeiting continues to be a threat. In response, several security features have been incorporated into modern paper currency.
Engraving
The most common security measure on U.S. money involves a level of image clarity that is difficult to replicate. The presidential portraits are finely detailed, giving them a lifelike appearance against the bill's background. The lines and scrollwork on the border pattern, Federal Reserve and Treasury seals are also clear and distinct.
Security Thread
In 1990, a polyester security thread was embedded into the $5 and $100 bills which can only be seen by holding bills up to a light source. The thread is a denomination identifier that runs through the seal on the left side; on a U.S. $20 bill, it will read "USA TWENTY."
Microprinting
U.S. currency features microprinting that cannot be easily reproduced. Located in various areas of a bill depending on the denomination and series, what appears to be a thin line on the currency's surface actually reads "United States of America" or "USA 100" on a $100 bill. This printing can only be seen under magnification and blurs when photocopied.
