An island's currency was often too big to move, so ownership just changed by word of mouth

On the Micronesian island of Yap, giant carved limestone discs called rai served as high-value money, some over 3 metres across, quarried on a distant island and paddled home by canoe. Owners rarely moved the stones at all; the island simply agreed on who owned which one, even a stone lying on the seafloor after a shipwreck. Economist Milton Friedman used Yap's stone money to argue that all money ultimately works the same way: by shared belief.

— Milton Friedman, Money Mischief: Episodes in Monetary History — Chapter 1, 'The Island of Stone Money', Harcourt Brace, 1994
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