Machine generated contents note: 1.Why is there money? -- 2.An economy without money -- 3.The trading post model -- 4.An elementary linear example: liquidity creates money -- 5.Absence of double coincidence of wants is essential to monetization in a linear economy -- 6.Uniqueness of money: scale economy and network externality -- 7.Monetization of general equilibrium -- 8.Government-issued fiat money -- 9.Efficient structure of exchange -- 10.Microfoundations of Jevons's double coincidence condition -- 11.Commodity money equilibrium in a convex trading post economy -- 12.Efficiency of commodity money equilibrium -- 13.Alternative models -- 14.Conclusion and a research agenda.