I visited the Money Museum at the Denver Federal Reserve the other day with a friend and on display they had Revolution era paper money. Printed quite clearly on each of them said that the bearer could exchange the paper currency for Spanish milled silver. Why is that? Was Spanish silver the prime choice for an international currency?
Spanish milled dollars were (for the time) an exceptional standard of weight and quality that were widely accepted and well distributed in the Americas before the North American colonies were founded. This, coupled with the fact that British mercantile policies constantly left the American colonies short of British coin, led the colonies to prefer the piece of eight, which was still legal tender in the US into the 1850s.
Encyclopaedia Britannica, 14th Ed. Articles on Peso, Dollar, Mint, Numismatics