[sabatonfan]

Is this something strictly with the US currency notes or is the same true for other countries currency as well?

[SaberTail]

It's most notes, and for EU and US notes (as well as some others), it's based on a certain pattern on the bills: https://en.wikipedia.org/wiki/EURion_constellation

[mlmonkey]

Of course, countries with plastic currency notes (like Australia) don't have to worry about this because their notes incorporate hard-to-counterfeit features like transparent windows.