Question

define double coincidence ?

Answer 1

This occurs when two people have goods they are both happy to swap in exchange. i.e. a perfect barter exchange.

If you two individuals place equal value on 4 eggs and a loaf of bread. Then this exchange would be a double coincidence of wants and enable an efficient transaction.

In a barter economy (which has no money) people have to swap goods. E.g. you pay me in eggs and I teach you economics. Clearly, a barter economy has significant limitations. It requires two people to have goods they are willing to swap.

Answer 2

The coincidence of wants problem (often "double coincidence of wants" [ verification needed ]) is an important category of transaction costs that impose severe limitations on economies lacking a medium of exchange (such as money), which have to rely on barter or other in-kind transactions.