Marx: "Yesterday I pawned a coat dating back to my Liverpool days in order to buy writing paper" (Karl Marx and Frederick Engels, Collected Works, vol. 38 [1852-55]: 221).

On the significance of Marx's coat, see Peter Stallybrass, “Marx’s Coat,” in Border Fetishisms: Material Objects in Unstable Spaces, ed. Patricia Spyer (New York: Routledge, 1998): 183–207. [PDF].

Exchange value appears as the property of a commodity that is exchangeable for other commodities. It also presupposes societies who produce commodities and exchange them. While all societies have things with use values, exchange value is relative to a specific time and place.

Additionally, exchanging commodities must also presupposes a way to determine proportionality between different commodities, so that they can be exchanged in the first place.

Exchange therefore requires some other measure that stands above the two commodities meant to be exchanged. If there were no ways in which iron and corn were found similar to a society, for example, then we would not exchange them and they would have no exchange value.

Marx will contend that what each commodity must contain crystalized within it is value (formally) and that the substance of value is labor (viz. the common factor of both iron and corn is labor). Marx will call this kind of labor abstract labor.