An 1881 box diagram shows exactly when two traders should stop haggling

Francis Ysidro Edgeworth's 1881 Mathematical Psychics plotted two people's preferences for two goods inside a single rectangle. Any trade both sides prefer pulls them toward a diagonal 'contract curve,' where no further swap can help one person without hurting the other. The 'Edgeworth box' became the standard textbook diagram for teaching general equilibrium and Pareto efficiency in exchange.

— Francis Ysidro Edgeworth, Mathematical Psychics: An Essay on the Application of Mathematics to the Moral Sciences — 1881
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