How one box can show two people’s choices
An Edgeworth box is a diagram for a simple economy with two people and two goods. The box’s width and height represent the total amounts available of each good. One person’s holdings are measured from the lower-left corner; the other person’s are measured from the opposite corner. So every point in the box describes a complete division of the goods between them.
Each person’s preferences can be drawn as indifference curves: lines connecting allocations that person regards as equally desirable. Curves farther in a person’s preferred direction represent better outcomes. The two sets of curves make it possible to compare what each person gains or loses when goods change hands.
The overlap marks a possible gain
Start at the point showing the initial allocation—what each person owns before trading. If there is an area where both people’s preferred directions overlap, a move into that area gives each a bundle they like better than their starting bundle. That region is called the lens of mutually beneficial trades. The box does not say that every exchange helps both sides; it shows when an exchange could.
A move may benefit one person while leaving the other no worse off, too. Economists call such a change a Pareto improvement. When no feasible rearrangement can make someone better off without making the other worse off, the allocation is Pareto efficient. The set of these allocations forms the contract curve.
What the diagram leaves open
The box identifies room for gains, but it does not decide who should receive how much, or guarantee that people will reach an efficient deal. Bargaining power, information, and the rules of exchange matter. Its useful lesson is narrower: with fixed resources, an initial division can leave both people able to gain simply by swapping goods they value differently.
