Tablebase

An endgame tablebase contains information calculated in advance for positions with few pieces. Instead of estimating a position, it can report what result can be forced with perfect play within its coverage, usually a win, draw, or loss for the side to move.

Tablebases are generated exhaustively by working backward from known final positions. Syzygy tablebases, widely used by chess engines, cover material with up to seven pieces. Their WDL files record win, draw, or loss. Their DTZ files record distance to zero, meaning the distance to a move such as a capture or pawn move. Those moves reset the count relevant to the fifty-move rule, which can allow a draw claim after fifty moves by each player with no capture or pawn move. DTZ information helps select play compatible with that rule.

A tablebase is not simply an extremely deep engine evaluation. For a covered position, the result comes from precomputed data rather than from increasing normal search depth. It is also different from an opening book: a book supplies prepared choices near the start of a game, while a tablebase solves endgames with limited material.