What is double coincidence. Give examples

Double coincidence of wants occurs when two individuals swap (exchange) their goods, in exchange for one another. This is also referred to as the 'perfect barter exchange'. In such cases, both the individuals are happy to exchange their good or commodities. 

Examole:

A shoe manufacturer wants to sell shoes in the market and buy wheat. The shoe manufacturer
will first exchange shoes that he had produced for money, and then exchange the money for wheat. Imagine how difficult it would be if the shoe manufacturer had to directly exchange shoes for wheat without using money. He would have to look for a wheat growing farmer, who not only wants to sell wheat but also wants to buy the shoes in exchange. That is, both parties have to agree to sell and buy each other’s commodities. This is known as double coincidence of wants.