How God plays chess

by Frederic Friedel
1/23/2018 – In the early 1980s Ken Thompson, working at the Bell Laboratories, generated one of the world’s first chess endgame databases — king and queen vs king and rook. At the time he explained to Frederic Friedel how this revolutionary new technology worked. He did it in the form of a parable: God calculating the 32-piece endgame and playing chess. It is an amusing thought experiment that has gained interesting relevance at the current time.

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In the beginning, the four-piece ending

It was some time in the early 1980s. Ken Thompson, the man behind Unix and the computer language C, had built a hardware chess machine named Belle, which duly won the World Computer Chess Championship. Ken had also resolved a dispute with IM David Levy on the feasibility of generating endgame databases — by actually generating a four-piece database, king and queen vs king and rook. Soon after it was completed, we had a lot of fun, at a tournament in New Jersey, challenging top players to win with the queen. "But that is quite easy," they would say, only to fail again and again against the optimum defence of the computer. One prominent victim: GM Walter Browne, three times US Champion.

After one of the rounds in New Jersey we sat in Ken’s car waiting for someone who would be getting a ride. It took far longer than expected, and I used the opportunity to elicit more information on how his "endgame databases" were constructed. Ken explained: you generate every legal position with the given material — for KQ-KR there are 1.9 million of them — and store them on a hard disk. Then a program examines each position to see if one side is mated. If that is the case the position is marked with a zero. Now it retracts one legal move from all positions that are marked zero and marks them with a one. These are positions that allow one side to deliver mate in one move.

Now the program takes all positions marked with a one and retracts all legal moves from each of them. It checks these positions for ones in which every legal move leads to a position that was previously marked. Such positions are marked with a two. They are positions in which the side to move will be mated by force in two ply (half-moves).

In the above schematic, taken from the German language book Steinwender/Friedel, Schach am PC, Markt&Technik 1995, positions with White to move are depicted as circles, Black to move as squares. In the first diagram positions in which Black is mated are marked zero; in the second all positions in which White can mate in one move are marked with a one; and in the third diagram all positions with Black to move that lead only to positions marked one are marked two.

The process is continued until all 1.9 million positions have been processed. With the hardware available at the time (a mainframe in Bell Labs, New Jersey) it took half a day to process the endgame. The result: over 1.7 million positions got a number between 0 and 61, which means that the maximum length of a win in the endgame queen vs rook is 31 moves. Since more than 90% of all positions are marked, this endgame is considered a win for the side with the queen. It is interesting to note that there are exactly two positions that require the maximum number of moves. One of them is wKa8, Qa5, bKf6, Re8, the position that Walter Browne could not win in 50 moves.

But what about the 10% of unmarked positions? Many are "pathological positions" in which Black can exchange the rook for the queen, e.g. wKb1, Qa1, bKb8, Rh1, which is an immediate draw (1.Kb2/Ka2 Rxa1). In fact Black wins if the white king is on d1, e1 or f1. But there are also more complex draws. The following has been known for over two hundred years: 


The rook keeps checking White from h7, g7 and f7. If the white king moves onto the e-file then …Re7 pins the queen and wins it; and if the king moves to f6 or h6 then …Rg6+/Rh7+ forces stalemate.

Peter Leko in 1992

Before we proceed with the narrative, there are two little stories I need to insert. They have been told before, but many readers may not be familiar with them. The first is that, at a tournament in Brussels, I got Karpov and Kasparov to sit together in front of my Atari ST and try to win queen vs rook. It was a hurried attempt in the press room after a round. They did not succeed, and laughed in amazement over the computer’s defence.

The other is about a 14-year-old chess talent who stayed at my house and whom I asked if he could win with the queen vs the rook. "Of course I can," he said, only to fail against the computer.

The next morning at breakfast he announced he had found the strategy to win, and proved it with flawless play against the computer. The lad, whose name was Peter Leko, had actually worked it out in his head, while lying in bed in the dark.

(Above right) Peter Leko in 1992 | Photo: GFHund CC BY 3.0 via Wikimedia Commons

How God plays chess

Back to the parking lot in New Jersey. I was very excited by the way the computer had stood up to GMs and IMs in the four-piece endgame, and eager to understand what endgame databases meant for the game. “When computers have calculated the 32-piece ending they will play 1.e4 and announce mate?” I asked. “No!” answered Ken emphatically, and proceeded to explain in a parable. I wrote this up a couple of years later, for a German chess magazine, and drew a fair bit of consternation and ire over it. At the time Garry Kasparov had ascended to the top of the chess world, and I inserted him into Ken’s narrative. Anyway, here goes…

One day World Champion Garry Kasparov, who towers above all his rivals, reached an Elo of 3000. When the rating list was released the heavens over Baku opened and an Angel of the Lord descended. He approached Garry and said: “For what you have achieved you are invited to play a game of chess against God.”

Garry was overwhelmed. He dressed into his finest and got onto the golden escalator that transported him to heaven. There the Angel led him into a small room where God was sitting, drinking coffee and looking at a computer screen. Garry was somewhat surprised that He was wearing jeans and a t-shirt, and had a fairly unkempt white beard. He bowed deeply and said:

“Oh Almighty Lord, Creator of the Universe…”

“Just call me God,” God interrupted. “Or G, which is what most of my friends call me.”

“Okay, God, it is a great privilege for me to stand in your presence and to actually play a game of chess against you. Of course I have absolutely no expectation of winning. I assume you play a perfect game!?”

“Yes,” replied God, “I have done the 32-piece endgame.”

“Ahh,” said Garry, “Of course that is trivially easy for you.”

“No, no,” said God, “it was really tough. More than 10^35 legal positions — it took the matter from a good-sized planet to store. But let us play. You can have white.”

So Garry played his favourite 1.e4, expecting an exciting Sicilian. But after a few moves the game had left all known theory and a weird position was on the board. Actually God’s position was in shambles. "This is divine humour," Garry said, "You want me to have a chance, God?" But He just smiled and said "No." Soon Garry was two pawns up and had an overwhelming position. "But I am winning for sure now," he said. "No," said God, "it is a draw. The whole game is a draw. Can’t be won."

Garry was not convinced. He played on with great attacking moves, and even won an additional piece. Surely now he was completely winning. He reached a stage where it was clearly only a matter of technique to mate the opponent. However, try as he might, he was not able to actually deliver mate. There was always some unexpected defence, some bizarre move that prevented it. He tried this and that, but somehow could not succeed.

Then suddenly God interrupted a phone call he was making and stared in disbelief at the board. "My Self!" he said, "It’s a win! I haven’t seen one in a billion years." "But I can’t find it," said Garry, "I cannot seem to convert my advantage to a mate." "Oh, no," God said, "it is a win for me — for Black. This only happens once in a decillion times."

The game proceeded, and God, now fully concentrated, started making moves that were completely incomprehensible to Garry. But they slowly improved Black’s position. Soon the game was equal, and then Black started to take what for Garry seemed to be the "initiative". Then, in a flurry of unexpectedly brilliant moves, he found himself mated.

"I will be eternally grateful, God, for this demonstration of your omniscience," Garry said in parting. "I thank you for a very interesting game, Garry," God replied. "A one in a billion experience."

So what was Ken telling me in that carpark? First of all that calculating the 32-piece ending, i.e. comprehensively solving the game of chess, was beyond the scope of the current universe. "Just storing the data requires you to dismantle an entire planetary system," he said. "Just imagine what Greenpeace would say to that!"

Secondly Ken was predicting that the game of chess was a draw, i.e. that probably a majority of all legal positions, including the starting position, would lead to a draw with perfect play. Both sides can always play moves that lead to unmarked (drawing) positions. He also drew my attention to a problem he had with his four and later five-piece endings: if the computer had a position that was a draw it would not try to win. It sometimes sacrificed its superior material because that did not change the outcome of the game. In the Ponziani position given above his computer would be willing to exchange the queen for the rook at any stage, because it did not affect the final result.

In the meantime all five, six and seven-piece endings have been calculated, and stored in databases of five hundred trillion positions. Of course the play of these endgame programs is completely incomprehensible, but Ken’s prediction that the game itself, in the 32-piece version, is probably a draw, seems plausible. And this is further confirmed by the recent experiment with AlphaZero. The system achieved super-human chess skill after just four hours of "thought", probably achieving something in the range of a 3700 rating. So the question arises: what happens if you let the system ponder for eight hours, or for eighty, or run on a far larger number of processors? My prediction: it will not cross Elo 4000 — nothing ever will. The draw in chess will prevent that from happening: a 3900 program will always be able to hold the game, however strong the opponent.

But this is a discussion for a future article, which will include the opinions of leading experts and hopefully some of those from our more thoughtful readers.

Endgame databases for notebook computers

In the years after the pioneering work with four-piece endings, Ken Thompson and others worked out all five-piece endgames, which have between 212 and 335 million positions each. And then came the six-piece endings, which required truly massive computing power to calculate — and plenty of hard disk space to store. Russian programmer Eugene Nalimov created a new format that required eight times less space than the previous versions. And in 2013, Ronald de Man developed the "Syzygy Bases" that were in turn seven times smaller than the Nalimov tablebases. For the first time it was possible (or practical) to provide users six-men tablebases on DVDs.

If you buy one of our chess programs, they will already play a number of four and five-piece endings (e.g. Q vs R or R+P vs R) perfectly. With Endgame Turbo the programs will play all important five and six-piece endings perfectly. They will in fact use the endgame knowledge in the search, so that positions with many more than six pieces that can be traded down to advantageous six or five-piece endings will be handled perfectly as well.

Endgame Turbo 4 contains all five and 27 of the most important six-piece endgames in the Syzygy format, which can be used with top engines like Komodo or Houdini. Endgame Turbo 4 consists of four DVDs with the following endgames:

You can order Endgame Turbo 4 in the ChessBase shop here.
Price: €59.90 – €50.34 without VAT (for customers outside the EU) and $54.03 (without VAT).

Expect to read more about five and six-piece endgame databases in a future report.


Editor-in-Chief emeritus of the ChessBase News page. Studied Philosophy and Linguistics at the University of Hamburg and Oxford, graduating with a thesis on speech act theory and moral language. He started a university career but switched to science journalism, producing documentaries for German TV. In 1986 he co-founded ChessBase.
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Keshava Keshava 12/16/2018 10:26
I would not insult their intelligence by suggesting that they have bungled anything since they have gotten the result that they wanted. Of course they could have easily consulted with the programmers of the Stockfish project or just followed protocols used by ccrl or tcec. This is interesting: "Similarly, some shogi observers argued that the elmo hash size was too low, that the resignation settings and the "EnteringKingRule" settings may have been inappropriate"
celeje celeje 2/5/2018 10:36
Another reason to stick to tablebases and not take Alpha Zero too seriously ---

Kai Laskos tested Stockfish against itself modeling optimal hash size against bad hash size given in Alpha Zero match.
Result was +189 -66 =745 (+43 Elo points).

So even just hash size bungling can causes a big effect.

Lesson: Stick to tablebases.
celeje celeje 2/4/2018 10:34
Frederic, I'd like to hear Ken Thompson's views about what he thinks is the maximum size endgame tablebase that can be worked out in the future, since only tablebases can say anything about perfect play.
celeje celeje 2/4/2018 06:22
wunderbare Geschichte, Frederic. Aber hüte dich vor Deepmind-Werbung.

GM Boros judged Alpha Zero's play as 2800 on the human elo scale.

Note: the computer ratings are not matched to the human ratings.

If you just swallow the match result, then it's roughly 100 elo more than whatever it's badly misconfigured opponent was. Maybe 3000??

@ fgkdjlkag: yes. I don't know why myths keep being repeated. When they finally stopped training it after 9 hours, it hadn't shown any improvement for many hours.
fgkdjlkag fgkdjlkag 1/25/2018 10:26
The idea of 4 hours and alphazero has been debunked.
Bobbyfozz Bobbyfozz 1/24/2018 07:03
I completely enjoyed Friedel's story and explanations. I have been healing for the last two weeks due to back surgery and have played no chess (!) and what a relaxation that was. Now I am reading again and Fred's story was most entertaining.
However (not a true "but,") why would the omniscient being (God) take an advantage to force an opponent (who also had "unlimited time") to do something dumb just to win. God (the omniscient being) would know before the contest even began whether White, or Black, would win or draw. Sometimes it is important to understand (whether or not one is a believer) what the word "mniscient" truly means. Great piece!
jjmolina jjmolina 1/24/2018 12:55
@Frederic Friedel
Comparing Nalimov and syzygy TBs isn't straightforward; while the first ones are "real" TBs, in the sense that they provide complete information for every position contained ('DTM' for every move won/lost or draw score), what Ronald de Man developed was essentially 6-men bitbases ('WDL' during search and the most basic info needed for 'DTZ50', at root). Other bitbase implementations already existed, Triplebases being the first ones to support 6-men.

BTW, anyone interested should know that syzygy bases are free and available for download as a complete set at olympuschess (unlike the DVDs, which only have a portion of them).

@Bojan KG
God would destroy AlphaZero, because it could make it fail, same thing if it played against a top centaur, the human component could always suffer an inconvenient health issue; but other than resorting to tricks, I don't see how any entity could win a game of chess, when in practice it has been a draw at the highest level, for at least a couple of years.

I don't know about the "majority of all legal positions", but if chess is a draw (it is) then it follows (not obviously but logically) that "the starting position, [will] lead to a draw with perfect play”.
Mark S Mark S 1/24/2018 09:54
Very nice illistrative picture. Thanks for this nice article and great photo.
Also noticed that if you stare at the dark sky (on the photo above), then slide your eyes going down the water area, the rock at the right hand side seems to move toward the center. This is some kind of optical moving illusion.
MicroTone MicroTone 1/24/2018 03:09
While it seems that, in fact, “the game of chess [is] a draw”, it obviously doesn’t follow (and it’s almost certainly not true) that “probably a majority of all legal positions, including the starting position, [will] lead to a draw with perfect play”. I seriously doubt Thompson believes that and I think this is just a misinterpretation by Friedel.

I understand your argument but just think about it: if you randomly throw pieces at a chessboard, the odds that you'll end up with a highly unbalanced position (three queens against two, with no immediate mates or perpetuals) are far greater than getting a balanced one.
Bojan KG Bojan KG 1/23/2018 10:40
Chess is so complicated - many moves made by AlphaZero in a match vs SF were simply out of this world and none human would play these in a million years. Speaking of God playing chess, of course God would destroy AlphaZero 100:0 in 100 games match by complicating positions to the very limits when machine would start to crack. Just to note here, number of (legal) positions in 32-piece endgame should be much higher than 10^35?
macauley macauley 1/23/2018 12:20
@Zvi Mendlowitz - The solutions to the Benko puzzles will be published on Thursday. Thanks!
Zvi Mendlowitz Zvi Mendlowitz 1/23/2018 09:23
Speaking of computers and chess: the Pal Benko puzzles were supposed to be the beggining of the christmas week puzzles, consisting of puzzles that cannot be solved by computer. But those puzzles never appeared...
geraldsky geraldsky 1/23/2018 04:21
K+Q vs. K+R is one the ugliest endgame ever in chess. In actual games most were ended in draws than wins , because most players (even masters) never knew how to play it accurately.
SmartShark SmartShark 1/23/2018 03:52
Interesting article. We humans struggle to find the right moves on an 8 x 8 chessboard. What if it were an 800 x 800 chessboard or an 8000000 x 8000000? Now imagine if we watched God play on this 8000000 x 8000000 chessboard and asked him to explain his moves! How long would it take for us to understand? Could we possibly understand? I suppose not. We would simply have to trust him when he says "Be31579 is the winning move here"! And so it goes with life as well :)
KWRegan KWRegan 1/23/2018 01:31
Schamlose Selbstwerbung: my article addresses a few questions raised here.