4. In the simplest economies people barter—trade goods and services for one another—rather than trade them for money, as in a modern economy. The circular-flow diagram represents transactions within the economy as flows of goods, services, and money between households and firms. These transactions occur in markets for goods and services and factor markets, markets for factors of production—land, labor, capital, and human capital. It is useful in understanding how spending, production, employment, income, and growth are related in the economy. Ultimately, factor markets determine the economy’s income distribution, how an economy’s total income is allocated to the owners of the factors of production.