Currency is also unique in that it supplies the unit of account for the whole economy. Although privately supplied inside money and publicly supplied outside money compete with one another as transaction media and stores of value, there is nevertheless a kind of symbiotic relationship between the two.
This is, first, because inside money (predominantly bank deposits) may owe its acceptability to the fact that its issuers promise, and indeed are legally bound, to redeem it [either immediately (sight deposits), upon maturity (time deposits) or after a period of notice (savings deposits)] in outside money and, second, because outside money provides the unit of account for the whole system without which inside money might be less acceptable in the absence of a single (commodity) numeraire.