Production is characterized by opportunity costs when producing one good involves sacrificing some amount of another good. The shape of the resulting production possibilities frontier (PPF) depends on whether these opportunity costs are constant or increasing.

• When opportunity costs are constant , the PPF will be a straight line. This linear shape occurs because resources are perfectly adaptable, meaning they can be substituted between the production of two goods without any added cost. The opportunity cost remains the same regardless of the quantity produced. For example, goods that require similar resources, like bread and pastry, tend to have such straight-line PPFs.
• When opportunity costs increase , the PPF is bowed outward (concave). This reflects that some resources are better suited for producing one good over another, so shifting production results in increasingly larger sacrifices of the other good. This is the more common case, where opportunity costs vary depending on the mix of production.

In summary, if the production is characterized by constant opportunity costs, the production possibilities frontier will be a straight line because the trade-off between the goods produced remains unchanged as production shifts from one good to another.