When was the watermark put on the US dollar bill?

The watermark on U.S. dollar bills was introduced with the redesign of the $20 bill in 1998 and has been a feature on various denominations since then, including the $100 bill in 2013. The watermark is a security feature intended to help deter counterfeiting and is visible when the bill is held up to the light. Prior to this, watermarks were used in earlier designs, but the modern versions are more sophisticated and prominent.