Endgame tablebase

In chess, the endgame tablebase, or simply tablebase, is a computerised database containing precalculated evaluations of endgame positions. Tablebases are used to analyse finished games, as well as by chess engines to evaluate positions during play. Tablebases are typically exhaustive, covering every legal arrangement of a specific selection of pieces on the board, with both White and Black to move. For each position, the tablebase records the ultimate result of the game (i.e. a win for White, a win for Black, or a draw) and the number of moves required to achieve that result, both assuming perfect play. Because every legal move in a covered position results in another covered position, the tablebase acts as an oracle that always provides the optimal move.

Tablebases are generated by retrograde analysis, working backwards from checkmated positions. By 2005, tablebases for all positions having up to six pieces, including the two kings, had been created.^[1] By August 2012, tablebases had solved chess for almost every position with up to seven pieces, with certain subclasses omitted due to their assumed triviality;^[2][3] these omitted positions were included by August 2018.^[4] As of 2024^[update], work is still underway to solve all eight-piece positions.

Tablebases have profoundly advanced the chess community's understanding of endgame theory. Some positions which humans had analysed as draws were proven to be winnable; in some cases, tablebase analysis found a mate in more than five hundred moves, far beyond the ability of humans, and beyond the capability of a computer during play. This caused the fifty-move rule to be called into question, since many positions were discovered that were winning for one side but drawn during play because of this rule. Initially, some exceptions to the fifty-move rule were introduced, but when more extreme cases were later discovered, these exceptions were removed. Tablebases also facilitate the composition of endgame studies.

While endgame tablebases exist for other board games, such as checkers,^[5] nine men's morris,^[6] and some chess variants,^[7] the term endgame tablebase is usually assumed to refer to chess tablebases.