Forms of money

The idea that anything can be money if we all just believe hard enough in it, has been very popular among some regimes and politicians. Yes, of course, anything can be used as money, just like anything can be used as toilet paper, but choices have consequences. — Saifedean Ammous. 

Free market money

Its creation is not monopolized, its adoption is voluntary.

Examples: collectibles, rai stones, gold or bitcoin.

Fiat money

The word "Fiat" originates in Latin, meaning decree, order.

The currencies issued by central banks are fiat money, like the US dollar or euro.

It is imposed by the state, usually by decrees that establish legal privileges like legal tender laws.

Legal tender is a medium of exchange recognized by law to be valid for extinguishing any debt when offered ("tendered"), unless there is a specific agreement to the contrary.

The legal tender legislation also implies that it is required as an Unit of Account for the calculation of private

incomes and balances, from which taxes are derived. This creates uncertainty when using a non-official currency because of the exchange rate fluctuation.

It is also the Unit of Account in which any State contract is denominated. 

To guarantee a privileged distribution when it is first introduced, the exchange of the previous form of money, such as the exchange of gold for US dollar in 1933, may be mandatory.

Its distribution is also benefited by its use as payment for public expenses. 

This privileges the adoption of the legal currency and, therefore, its saleability; which implies forced benefits in its function as a Medium of Exchange.

The rulers have the monopoly of production by the enforcement of anti-counterfeit statute, which allows them to extract seigniorage (an indirect tax on money adopters): the difference between its purchasing power and its production cost.