A production possibilities frontier (PPF) is a straight line when opportunity costs are constant between the two goods being produced.

What this means

  • A straight‑line PPF implies that shifting resources from one good to another does not change the opportunity cost.
  • This usually happens when resources are not specialized and can be substituted freely between the two goods with no extra cost or loss in efficiency.

Simple example

Imagine an economy that produces only bread and pastries , using the same mix of labor, ovens, and flour.

  • Each extra loaf of bread costs the same amount of pastry, and vice versa.
  • The PPF will be straight because the trade‑off is constant , not increasing.

Straight vs bowed‑out PPF

Shape of PPF| Opportunity cost| Underlying idea
---|---|---
Straight line| Constant| Resources are easily and fully substitutable between goods. 57
Bowed‑out (concave)| Increasing| Resources are specialized; shifting them raises the opportunity cost. 59

So, in short:
“A production possibilities frontier is a straight line when opportunity costs are constant.”