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A party’s BATNA is the allocation that is exactly as good for them as walking away with no deal.

Edgeworth box: reserve positions and the zone of possible agreement

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A’s BATNA B’s BATNA Proposal Contract curve ZOPA B does best A does best A B

Would each party accept?

Party A
Party B

Source: F. Y. Edgeworth, Mathematical Psychics: An Essay on the Application of Mathematics to the Moral Sciences, London, C. Kegan Paul, 1881; the box form of the diagram is generally attributed to Pareto, Manuale di economia politica, 1906.