Commodity money cans or foil pouches of mackerel fish fillets, which have a fairly standard cost and are easy to store. These may be exchanged for many services in prisons where personal possession of currency is prohibited. In situations where the commodity is metal, typically gold or silver, a government mint will often coin money by placing a mark on the metal that serves as a guarantee of the weight and purity of the metal. In doing so, the government will often impose a fee which is known as seigniorage. The role of a mint and of coin differs between commodity money
Commodity (Marxism) of value, product-values in an open market are regulated by the average socially necessary labour time required to produce them, and price relativities of products are ultimately governed by the law of value. Commodity-trade, Marx argues, historically begins at the boundaries of separate economic communities based otherwise on a "non-commercial" form of production. Thus, producers trade in those goods of which those producers, have episodic or permanent surpluses to their own requirements, and they aim to obtain different goods with an equal value in return. Marx refers to this as "simple exchange" which implies what Frederick Engels calls "simple commodity