Euro notes are issued by the Central Banks of each of the member countries of the eurozone. However, although euros are legal tender in all of the eurozone, not all euros are identical. All euro banknotes have a one-letter, eleven-digit serial number on the back at the top right and bottom left corners of every denomination. The letter indicates the country that issued the banknote, which may determine the winners and losers in the event of a break-up.

Critics of the single currency have always pointed out the fundamental weakness that it lacks a single government or tax-raising authority to guarantee each euro remains worth more than the paper it is printed on. Indeed, some euros might be more widely accepted than others if the economic crisis takes a turn for the worse and the euro is abandoned, depending on which country issued them.

Although, all euros are backed by the European Central Bank, the serial numbers prefixed with X may be regarded as the most secure, because they are issued by Germany. N is also a good prefix, since these come from Austria. P, L, U and Z prefixes may also be favoured, because these are issued by the authorities in Holland, Finland, France and Belgium.

If the euro does not survive in its present form, it may be better to avoid notes with the prefixes F, G, M, S, T or Y as these are issued by Malta, Cyprus, Portugal, Italy, Ireland and Greece respectively. In the event of a breakdown of the euro it is suggested that X prefixed euros may become marks, while Y prefixed notes would become drachmas, and so on. This would mean that German euros might be worth two or three times as much as Greek euros.