A peer-to-peer exchange mechanism in which two parties are exactly matched. For example, A has two pigs and needs a cow. B has a cow and needs two pigs. There is some debate as to whether barter was the first method of exchange. For example, David Graeber argues that the earliest form of trade was in the form of debit-credit. People living in the same village gave each other “gifts,” which by social consensus had to be returned in future by another gift that is usually a little more valuable (interest). People kept track of exchanges in their minds as it was only natural and convenient to do so since there is only a handful sharing the same village. Coinage comes into play many, many years later with the rise of migration and war, with war tax being one of the very first use cases.