Euro currency remains a work in progress on 20th birthday

The euro is about to celebrate its 20th birthday, but the countries that use it are still wrestling over how the shared currency should work and how to fix flaws exposed by the debt crisis that marred its second decade.

The euro was launched on Jan. 1, 1999, when 10 countries fixed their exchange rates to it and handed decisions on interest rates to the newly-founded European Central Bank. Euro notes and coins went into circulation three years later.

The shared currency was seen as a solution to the constant quarrels over exchange rates that had marked European politics after World War II and as a logical extension of the European Union's tariff-free trade zone. Britain, notably, opted out, but 19 of 28 EU countries use the euro.

The euro is credited with increasing trade between members. But countries have struggled to adjust to trouble after giving up two...