Three types of exchange rate regimes are possible and have existed at various times in various countries.

The clearest example is a common currency: the dollar in the U.S.; the euro that will shortly reign [throughout] in the common market. Almost identical is the balboa in Panama, which is interchangeable with the dollar one-to-one, and the currency boards in Argentina and Hong Kong, which are committed to creating currency only in exchange for a specified amount of U.S. dollars (one peso to $1 in Argentina, 7.8 Hong Kong dollars to $1) and to keeping dollar reserves equal to the dollar value of all currency outstanding. A pure gold standard is a variant of this type of regime.

The key feature of the currency board is that there is only one central bank with the power to create money—in the examples cited, it's the U.S. Federal Reserve System. Hong Kong has no central bank; Argentina does—but without the power to create money. 

Hong Kong and Argentina have retained the option of terminating their currency boards, changing the exchange rate, or introducing central bank features, as the Hong Kong Monetary Authority has done in a limited way. As a result, they are not immune to infection from foreign exchange crises originating elsewhere. Nonetheless, currency boards have a good record of surviving such crises intact. Those options are not retained by Panama, and will not be retained by the countries that adopt the euro as their sole currency.