Money
In most cases this is a system of paper notes and coins, but this is not necessarily the case and certainly in the past has not always been the case, with commodities being used as money by several societies in the past.
Note that there is actually less (physical) paper money than there is money in circulation however, as most of it today is kept in bookkeeping debts and credits - it may be that around 80% of money is actually represented in this way rather than being 'physically real' as it were.
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