In the case of euro, coins and banknotes of former national currencies were considered as legal tender from January 1, 1999 until February 28, 2002 (in some cases) even if their corresponding currencies had ceased to exist. Legally, those coins and banknotes were considered non-decimal sub-divisions of euro.
Euro coins and banknotes were awarded legal tender status on January 1, 2002. All coins and all banknotes are legal tender throughout the eurozone, even if coins have national marks of each State. Therefore, it is possible to find Irish coins in Greece and Finnish coins in Portugal, for instances.
Legally, the fact that a currency has legal tender status only means that it is not to be refused in payment of a debt; therefore (at least in England) even legal tender can be refused until a person is in debt. This means that vending machines and transport staff do not have to accept the largest denomination of banknote for a single bus fare or bar of chocolate, and even shopkeepers can reject large banknotes, but restaurants that do not collect money until after the meal is served would have to accept any legal tender.