When the economy is using most of its resources to make guns, such as at point F, the resources best suited to butter production, such as chef, are being used in the gun industry. Because these workers probably aren’t very good at making guns, the economy won’t have to lose much gun production to increase butter production by one unit. The opportunity cost of a unit of butter in terms of guns is small, and the frontier is relatively flat. By contrast, when the economy is using most of its resources to make butter, such as at point E, the resources best suited to making butter are already in the butter industry. Producing an additional unit of butter means moving some of the best gun makers out of the gun industry and making them butter makers. As a result, producing an additional unit butter will mean a substantial loss of guns output. The opportunity cost of a unit of butter is high, and the frontier is quite deep. In a nutshell, since the resources best suited to butter making are limited, the more resources are used in butter production, the less efficient the production will be; thus the frontier, as the quantity of butter produced increases, grows deeper and deeper suggesting that opportunity cost of butter in terms of guns grows higher and higher.