Explain Chess Engine Endgame Tablebase

An endgame tablebase (EGTB) is a massive precomputed lookup of every legal position with ≤N pieces on the board. For each position it stores the exact outcome under perfect play — win, loss, or draw — and often how many moves until the next capture, pawn move, or check (a zeroing event). Modern engines (Stockfish, Lc0 wrappers, etc.) probe Syzygy files at search time instead of searching deep endgames from scratch.

Concept

Search Alone vs Tablebase Probe

Deep search only

Heuristic eval + 40+ ply search may miss fortress draws, tablebase wins, or optimal conversion paths. CPU-heavy and still wrong in theory.

Tablebase probe

O(1) disk lookup returns perfect WDL/DTZ. Engine converts to search score, prunes losing moves, picks shortest wins — mathematically correct.

≤7Pieces (Syzygy standard)

WDLWin / Draw / Loss flag

DTZDistance to zeroing move

~150 GBFull 7-man Syzygy set

Coverage

How Many Pieces?

3K+Q vs K

4K+R+P vs K

5K+R+B vs K+N

6K+P endgames

7Full Syzygy max

Count includes both kings and all remaining pieces. 8+ piece positions are not in standard Syzygy — engines fall back to search + NNUE/heuristic eval.

Data

WDL — The Three Outcomes

Win — side to move wins with perfect play (may require many moves).

Draw — neither side can force a win (includes stalemate, repetition, 50-move).

Loss — side to move loses with perfect defense (opponent wins).

Precision

DTZ — Distance to Zeroing

WDL files (.rtbw / .rtwd)

Smaller. Answer: can I win, hold a draw, or am I lost? Enough for correct result and many cutoffs. Used heavily in fast play.

DTZ files (.rtbz / .rtzd)

Larger. Answer: minimum half-moves until capture, pawn push, or check — the shortest path to conversion. Needed for optimal play and cursed wins.

Pipeline

How an Engine Probes Tablebases

Position
≤7 pieces

→

Normalize
EP · castling

→

Hash lookup
in .rtb*

→

WDL / DTZ
+ best move

Check piece count

If total pieces > limit (e.g. 7) or files missing — skip probe, use normal search.

Probe WDL

Fast win/draw/loss. Losing moves can be pruned; winning side prefers moves that preserve win.

Probe DTZ (if available)

Among winning moves, pick lowest DTZ — shortest conversion. Handle cursed wins (win but 50-move draw) and blessed losses.

Convert to search score

Map WDL/DTZ to centipawn-like bounds (e.g. TB win score ≈ +199 − dtz) so alpha-beta and UCI info stay consistent.

// Simplified probe → score (Stockfish-style idea)
if (pieces <= syzygyProbeLimit) {
  wdl = probe_wdl(pos);
  if (wdl == WIN)  score = +20000 - dtz;   // mate-ish bound
  if (wdl == LOSS) score = -20000 + dtz;
  if (wdl == DRAW) score = 0;
  return score;  // cutoff — no subtree search
}

Files

Syzygy File Types

*.rtbw

WDL tablebase (without DTZ). Most common for analysis with limited disk space.

*.rtbz

Combined WDL + DTZ in one file. Preferred when you want shortest mates.

*.rtwd / *.rtzd

Compression variants (large pages). Same data, different on-disk layout for faster probing.

KQRvK.rtbw

Named by material (Kings + pieces). One file per material signature up to 7 men.

Integration

Where Probes Hook Into Search

♟

Root & internal nodes

Before expanding a subtree, check TB. A proven draw returns score 0 immediately; a proven loss returns a huge negative — massive node savings.

♔

Move ordering

Winning TB moves rank first; losing moves may be skipped entirely. DTZ breaks ties among several winning moves.

50-move rule awareness

Syzygy stores cursed wins (win but draw under 50-move) and blessed losses. Engines adjust conversion strategy accordingly.

UCI

GUI configuration

Set SyzygyPath (or equivalent) to the folder containing .rtb* files. Option SyzygyProbeLimit caps piece count (3–7).

Reference

Formats & History

Format	Era	Used by	Notes
Nalimov .emt/.emm	1990s–2000s	Old engines, Crafty	5-man standard; replaced by Syzygy
Gaviota .gtb	2000s	Some hobby engines	Alternative compression; less common today
Syzygy .rtb*	2013+	Stockfish, Komodo, etc.	6-man complete; 7-man widely available
Scorpio bitbases	2010s	Endgame bitbases	Some 6-man bitbase sets; Syzygy dominates

Setup & usage tips

Download Syzygy files from official mirrors (e.g. syzygy-tables.info) — start with 3–5 man if disk is limited
Point engine to one directory; all .rtbw/.rtbz files for that limit must be present for a material class
WDL-only is fine for correct results; add DTZ for shortest mates and analysis quality
In analysis, enable TB — in bullet games some users disable DTZ probes to save I/O latency
Tablebases assume correct en passant and castling rights — FEN must match stored positions

Explore the full engine stack

Tablebases plug into search and UCI — see how the rest of a modern engine fits together.

Search Algorithm UCI Protocol Engine Anatomy

Related: Search Algorithm · Transposition Table · UCI Protocol · All Blogs