It's not even a few existing designs... There is an exact pattern called the eurion constellation that all printers block. Then the process for printing each countries currency just involves placing a eurion constellation and you get the protection.

Its a single pattern that is very rigidly defined, to the point that many counterfeit bills that pass inspection shift one mark off by about a millimeter to get it printed.

Granted having worked at a bank when I found out was that the bill didn't have the off color threads as well, but it almost was approved despite getting a close inspection.