Consider the production possibilities frontier model for an economy that produces only two goods: barley and cars.

Production Possibilities Frontier

Explanation:

In a theoretical economy, the production possibilities frontier, is the curve that shows the combination of goods produced (barley and cars) by an economy given a limited resource. Furthermore the more goods (barley) is produced, the less cars are produced. Thus, for every additional barley's produced, there's an opportunity cost of cars.