All great empires – from the Greek, to the Roman, the Spanish and the British - have at their heart a dominant means of exchange which is very much part of their political and social hegemony. Once upon a time, it was Roman coinage which was the world's pre-eminent currency. In more recent times it was the British pound. Today, it's the US dollar to which international investors flock as a safe haven for their money. Highly liquid and apparently reliable – until recently at least – nothing else comes even remotely close to the greenback's dominant position in the international monetary system.