Static exchange evaluation

Static exchange evaluation, abbreviated SEE, answers a local question: if a series of captures begins on one square, what material result can each side obtain? Instead of searching every move on the board, it follows the attackers and defenders of that square.

The procedure commonly assumes that each side captures with its least valuable available attacker, because committing a cheaper piece first tends to be favorable. After every captured piece is removed, the attackers are recalculated. This matters because an x-ray attack may appear: a rook, bishop, or queen that was blocked becomes active when an intervening piece disappears.

Either side may also stop the sequence when continuing would worsen its result. By working backward through the imagined captures, SEE estimates whether the initial capture wins, breaks even, or loses material. Engines use this signal to prioritize captures in move ordering and to decide which captures deserve extension during quiescence search.

The word static signals the limitation. SEE focuses on a material exchange on one square and does not replace a complete tactical search. It may miss that a capture gives checkmate, creates a decisive threat, deflects a defender, or permits an in-between move elsewhere. A careful implementation must also handle pinned pieces, which cannot move without exposing their king, and attacks that cannot legally be used.