A unit of account is a standard numerical unit of measurement of the market value of goods, services, and other transactions. Also known as a "measure" or "standard" of relative worth and deferred payment, a unit of account is a necessary prerequisite for the formulation of commercial agreements that involve debt.

    * Divisible into small units without destroying its value; precious metals can be coined from bars, or melted down into bars again.
    * Fungible: that is, one unit or piece must be perceived as equivalent to any other, which is why diamonds, works of art or real estate are not suitable as money.
    * A specific weight, or measure, or size to be verifiably countable. For instance, coins are often made with ridges around the edges, so that any removal of material from the coin (lowering its commodity value) will be easy to detect.

Store of value

    Main article: Store of value

To act as a store of value, a commodity, a form of money, or financial capital must be able to be reliably saved, stored, and retrieved — and be predictably useful when it is so retrieved. Fiat currency like paper or electronic currency no longer backed by gold in most countries is not considered by some economists to be a store of value.

Market liquidity

    Main article: Market liquidity

Liquidity describes how easily an item can be traded for another item, or into the common currency within an economy. Money is the most liquid asset because it is universally recognised and accepted as the common currency. In this way, money gives consumers the freedom to trade goods and services easily without having to barter.

Liquid financial instruments are easily tradable and have low transaction costs. There should be no — or minimal — spread between the prices to buy and sell the instrument being used as money.

Types of money

In economics, money is a broad term that refers to any instrument that can be used in the resolution of debt. However, different types of money have different economic strengths and liabilities. Theoretician Ludwig von Mises made that point in his book The Theory of Money and Credit, and he argued for the importance of distinguishing among three types of money: commodity money, fiat money, and credit money. Modern monetary theory also distinguishes among different types of money, using a categorization system that focuses on the liquidity of money.