By Rodney Smith BBC World Service

The US dollar is probably the most widely recognised currency in the world - it's accepted universally and most internationally traded goods and commodities are priced in dollars.

The US Treasury calculates there are officially 659,160,111,810 dollars in circulation - or $660 billion in round figures.

But they can't find all of them. About four-fifths of the amount ever printed and accounted for has disappeared.

It's not hard to see why. For a start, more than half of it is not in the US, but in other countries.

Not American but Czech

When the world abandoned the gold standard 55 years ago, the Americans at the same time flooded war-ravaged western Europe and east Asia with dollars for reconstruction.

No one thought about recovering them, and they became a sort of external dollar, eventually evolving into the specialist eurodollar - not to be confused with the euro.

The biggest nightmare at the US Treasury was that one day someone would try to repatriate those billions of "foreign" dollars

So the dollar became a de facto world currency - several countries, particularly in South America, use it in parallel with their own own currencies - so-called dollarisation.

The origin of the name is not American either, but European. It comes from a small town in the Czech republic, Joachimsthal, that was famous for mining silver.

It minted coins called Joachimsthalers, and that was corrupted over time, and adopted by the US Congress after the American war of independence as the dollar.

Laundered

For many years, the biggest nightmare at the US Treasury was that one day someone would try to repatriate those billions of "foreign" dollars to the US.

That could swamp the country and ruin the economy. But it would be almost impossible to do in fact.

The biggest problem for the US, and the reason that dollars "disappear" is that the dollar has become the most prolific international currency. It's also fairly safe, so it's used everywhere - and especially by criminals and drug dealers.

It's up to the job alright. When the new euro note was introduced three years ago, London newspapers tried to find out which was the strongest currency.