The blue ribbon on a $100 bill is not a printed decoration; the U.S. Currency Education Program says it is woven into the paper, and its bells and 100s move when tilted

If you have a $100 bill in hand, look at that blue stripe down the middle of the bill. Most people do not take it seriously, as they think it is irrelevant and just present as a design. However, a U.S. Currency Education Program article says it is not printed directly onto the paper. It is woven into the paper and contains microscopic lenses that make the tiny bell images flip and slide when you tilt the bill. This is probably the most sophisticated tangible gadget that we all use without batting an eyelid in a generation that pays for a lot of things with just a phone tap.

The ribbon is doing more physics than your average gadget

The phenomenon you see when you tilt the bill is not an illusion; it's called moiré magnification. According to a 2013 peer-reviewed paper titled “High-resolution 1D moirés as counterfeit security features,” published in the journal Light: Science & Applications, it's the same optical effect as some anti-counterfeiting labels and ID cards, and it's achieved by overlaying a grid of microlenses and another matching grid of small printed images so the two patterns interact and seem to shift as you change your viewing angle. That's why security threads in several world currencies use this type of "moiré magnifier." It's a physics-based mechanism designed to prevent a flatbed scanner or photocopier from being able to reproduce the image accurately, the shifting effect only exists at the microscopic, angle-dependent level of the real ribbon, so a flat, two-dimensional scan or photocopy simply can't capture it.

The 3D ribbon appeared when a $100 note was redesigned completely, as was the patch of ink on the $100 figure that shifts from copper to green when you tilt it, and the faint watermark of Benjamin Franklin that becomes visible when the bill is held up to light. All three features were added together because the $100 is the most counterfeited U.S. denomination, not because of anything specific to Franklin's face, but because of the note's high value and the sheer volume in which it circulates, especially internationally. The redesign emphasized elements that are physically harder to mimic, rather than just harder to spot in the eyes of the counterfeiter.

So is any of this actually necessary anymore

Given the sophistication of such technology, you would expect there to be many fake hundreds. There aren't and that gap is really the point of the design, not a contradiction of it. According to a 2025 paper titled “Estimating the Volume of Counterfeit U.S. Currency in Circulation,” by economist Ruth Judson and published in the International Finance Discussion Papers under the Federal Reserve Board, the estimated amount of counterfeited dollars in circulation within the United States was about $15 million, compared to approximately $2.3 trillion in legitimate dollars in circulation, that is, about one fake dollar for every 80,000 genuine dollars. The paper adds that that number has been declining over the past several years, thanks in part to security enhancements such as these coupled with public awareness campaigns.

So, is the U.S. using a sophisticated micro-optics countermeasure against a relatively small threat right now? Perhaps. But that’s kind of the whole idea behind proper security engineering. You don’t let things become a significant problem before you solve them, and a $100 bill is circulated the world in person-to-person transactions in countries where digital authentication isn’t an option like on your smartphone.

Worth a second look next time you're holding one

For so many generations, paying with cash has become somewhat quaint. The next time you get a $100 bill circulates around the world in person-to-person transactions in places where digital authentication is not an option, unlike on your smartphone. You will not just be viewing currency; you'll be viewing a little, quiet, cotton-and-linen optical lab that's been effectively fooling counterfeiters since October 2013, and nothing needs to be added, taken away, or even updated, and no battery is required.