Take a look at the banknotes below. What do they have in common?

Did you spot it? Each of the banknotes above (and many, many others) include the EURion constellation somewhere in their design.

The EURion constellation is a pattern of five rings, arranged in a rough ‘X’ shape, used by banks to identify to scanners, photocopiers/printers and software that they should not reproduce the object being imaged. If you try to image a banknote that includes the EURion constellation on modern hardware or software you will generate an error, or print a blank or full-black page.

Some banknotes attempt to embed the EURion constellation into their design: for example, the Australian dollar above uses them as “leaves” on a tree to the right of Catherine Helen Spence’s head; and US dollar notes use the zeroes in the bill’s denomination (10, 20, 50, 100). The circles that make up the constellation are often printed in yellow, or another colour so that they blend in with their background.

The EURion constellation is not the only method used to protect banknotes, as Markus Kuhn, the discoverer of the EURion constellation has noted.