In a collaboration with Marc Bourzutschky, we can now expand Lichess's tablebase interface to a large subset of practical 8-piece positions. The tables are publicly available: for download (63 TiB); via the tablebase API for developers; and right on the analysis board and the mobile app.
Marc has a long history of contributions to endgame theory, including early work on 7-piece tables over twenty years ago. In 2021, he announced results from first explorations into 8-piece territory.
In 2025 Niklas Fiekas started working with Marc to bring the tables online.
The subset: Op1
The tablebase covers those 8-piece positions that have at least one white and one black pawn that oppose one another, impeding each other’s path to promotion. We refer to such pawns as an opposing pair, and to positions that contain at least one opposing pair as op1.
An opposing pair will be on one of the files a-h. For a given file there are 15 opposing pair positions. For example, on the a-file a white pawn on a2 and a black pawn on a3 form an opposing pair. Denoting this opposing pair by a2/a3, the remaining 14 opposing pairs on the a-file are a2/a4, a2/a5, a2/a6, a2/a7, a3/a4, a3/a5, a3/a6, a3/a7, a4/a5, a4/a6, a4/a7, a5/a6, a5/a7, a6/a7.
Opposing pairs are thus a generalization of the concept of blocked pawns. The key feature of an opposing pair is that neither of the two pawns in an opposing pair can promote unless there is an intermediate capture, even if the pair is far apart like a2/a7. By contrast, a3/a2 (white pawn on a3 and black pawn on a2) is not an opposing pair because each pawn might be able to promote without any intermediate captures.
In the tablebase we exclude extremely imbalanced positions where one side has just one pawn, i.e., endings like KQRBNPvKP. Most likely such lopsided positions are not very interesting.
Empirically, about half of all 8-piece endings reached in practical games are op1. This is confirmed by scanning game databases such as Chessbase’s megabase and the CCRL database of engine games. Indeed, slightly more than half of the 8-piece endgames reached on Lichess are op1, as shown in the graph below. So 8-piece op1 adds a good chunk to our existing 7-piece tablebase coverage via Syzygy.
Proportion of 3-8 piece endgames and respective op1 endgames reached on lichess.org in 2025
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