Edgeworth box: a graphical representation of all possible allocations of two goods between two people. Person A's origin is the bottom-left; Person B's origin is the top-right. The contract curve (Pareto set) traces all Pareto-efficient allocations, where MRS_A = MRS_B.
For Cobb-Douglas preferences U = x^α · y^(1-α), the contract curve is a straight line. Points inside the lens between the two indifference curves through the endowment represent Pareto improvements — both parties can gain from trade. The competitive equilibrium lies on both the contract curve and the budget line through the endowment.