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What is the longest chess game possible in terms of chess moves? I read somewhere that there is a theoretical maximum of 5949 moves. But I don't see any proof and I don't think it is correct.

Can it be infinite?

Source

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    There are rules about repetition of positions, and number of moves without a pawn move or a capture. This last rule has I believe been modified in the last $20$ years or so. – André Nicolas Nov 21 '13 at 7:00
  • Since this depends mostly on the applications of chess rules, I think it is better suited on the Chess StackExchange, and so I am migrating it there. @Justin: please register an account there so you can claim ownership to the question. – Willie Wong Nov 21 '13 at 8:25
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    A lot of answers consider the fact that the 50-move rule and the repetition rule do not take effect if neither player claims a draw. Now here is an interesting point: The new rule 9.6 in the FIDE laws of chess, valid from July 1st 2014, gives similar additional drawing rules (75 moves and 5 repetitions) which will take effect even if neither player claims the draw. (I think the point of this rule is to allow the arbiter to stop the game even if the players are for whatever reason delaying the end.) – JiK Nov 21 '13 at 17:34
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Some cleaning up is required, I think:

The number on the website you link to differs from the results published in Bonsdorff et al., Schach und Zahl. Unterhaltsame Schachmathematik. pp. 11–13. There they say that if the 50-move-rule is mandatory the longest possible game (i.e. where both players cooperate to achieve the weird goal of a game of maximal duration) lasts 5899 moves. Possibly, the web site used a simpler upper estimate for the "gaps" between pawn moves and captures that cannot be achieved in all occasions.

However, the 50-move-rule (and also the position-repeated-three-times-rule) is not mandatory, i.e. whether or not a player demands remis by that rule is up to him! The players may decide to ignore the rule and play on, thus allowing for an eventually periodic sequence of moves, i.e. an infinite game.

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    Please note that the FIDE rules in effect since 1 July 2014 do indeed specify mandatory limits (namely, the 75-move rule). – chaosflaws Apr 16 '15 at 13:12
  • @chaosflaws D'oh, this i show accepted answers become invalidateed over time ... :( – Hagen von Eitzen Apr 16 '15 at 17:50
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  1. 49 as black and white move knights.
  2. 32*50 = 1600; to lock the pawns up. In this case, White pushes each pawn 1 time until it is stopped by a black pawn.

  3. 6*50*8 = 2400; the white pawns are devoured one at a time, and as a black pawn is unblocked, it runs down the board, one square at a time. They promote to Knights.

  4. 7*50 = 350; each of the new knights are devoured.

  5. 30*50 = 1500; the rest of the pieces are devoured. Kings have to be left standing, so 30 here, not 31.

The sum of these moves is 5899. I don't know if it is the maximum, but it seems plausible.

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    6. King vs King is an automatic draw due to lack of materiel to cause a checkmate. Maximum is 5899. – Hans Z Nov 21 '13 at 17:02
  • Is that in the FIDE rules? – Tony Ennis Nov 22 '13 at 0:14
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    Article 9.6- The game is drawn when a position is reached from which a checkmate cannot occur by any possible series of legal moves. This immediately ends the game, provided that the move producing this position was legal. – Hans Z Nov 22 '13 at 2:21
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From Wikipedia (see http://en.wikipedia.org/wiki/Draw_%28chess%29):

"The rules allow for several types of draws: stalemate, the threefold repetition of a position (with the same player to move), if there has been no capture or a pawn being moved in the last fifty moves, if checkmate is impossible, or if the players agree to a draw. In games played under time control, a draw may result under additional conditions. A stalemate is an automatic draw, as is a draw because of insufficient material to checkmate. A draw by threefold repetition or the fifty-move rule may be claimed by one of the players with the arbiter (normally using his score sheet), and claiming it is optional."

So, if none of the players claims a draw, the game can go on forever. If at least one of the players intends to claim a draw when he has the possibility, then the threefold repetition rule and the fifty-move rule garantee that the game will end after a finite time. Maybe this can give the number of 5949 moves though? Considering the vast number of possible positions, the game could go on for much longer than 5949 moves before the threefold repetition rule applies. The fifty-move rule means that every 50 moves one of the players has to either move a pawn or make a capture. Pawn can make 2x8x6=96 moves. There are 32 pieces, so we can never exceed 50x(96+32)=6400 moves. So what is the minimal number of pieces that have to remain on the board to avoid a stalemate?

  • I did not go through your answer in detail, but at least one thing that you did not account for is that the pawns cannot reach the end of the board without making at least one capture. – Sid Nov 21 '13 at 7:33
  • And what does that change in my argument (which gives only an upper bound, anyway)? – UwF Nov 21 '13 at 7:43
  • Well if the pawns were to capture the opponent's pawns, then the number of maximum pawn moves would decrease. The pawns can therefore only capture pieces, are you sure this can happen while allowing all pawns to promote without a pawn standing in their way? Also note two things: i) the kings cannot be captured, so you can write 30 instead of 32 2) stalemate considerations are irrelevant, it can easily be avoided, especially as the number of pieces decrease. Furthermore, I think this question and discussion should be moved to chess.SE. – Sid Nov 21 '13 at 7:56
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    By stalemate he probably meant the insufficient material issue, that starts becoming a problem once there are very few pieces left on board. Some pawns need to make a capture, but not necessarily many of them, it would be difficult to estimate a better upper bound for this without a little bit of investigation, so I'd be happy to keep that number, but as you mentioned, writing 30 instead of 32 is a very simple improvement, so let's add this and calculate 50 * (96 + 30) + 49 (the first 49 moves can theoretically be just knight moves) = 6349. Add the pawns-needing-to-capture rule and voila! – downhand Apr 16 '15 at 6:21
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Ian Stewart discusses in an October 1995 Scientific American column how chess can be played with an infinite number of moves (and thus have a game that never ends).

Anyone who plays chess knows that some games just peter out : neither player seems able to win, nothing constructive can be done and there is no obvious way to end the game. If neither player agrees to a draw, the game might go on indefinitely. Foreseeing such situations, the bodies that frame the laws of chess have proposed many different rules to force games to end. The classic law states that the game shall be drawn if a player proves that 50 moves have been made on each side, checkmate has not been given, no men have been captured and no pawn has been moved.

But recent computer analyses have shown that the rule is not sufficient. There are some endgames in which one player can force a win after 50 moves, when no pieces have been captured and no pawns moved. So the laws of chess must specify certain exceptional situations. Any law that limits the number of moves permitted under particular conditions runs the same risk as the original, and so it would be nice to come up with a different approach altogether. One proposal, made some time ago, was that the game should end if the same sequence of moves, in exactly the same positions, is repeated three times in a row. (Do not confuse this with the standard law that if the same position occurs three times, the player facing it can claim a draw. But note that this law does not oblige them to do so.)

Stewart then proceeds to create a sequence of two symbols that never repeats a pattern thrice. He then shows that this sequence can be used by the two players to play a valid endless game even if the proposal becomes official. (This sequence is called Stewart's choral sequence.)

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    While this particular sequence is original to Stewart, of course the concept isn't; the well-known Thue-Morse sequence also satisfies the constraint that no sequence repeats three times (though the proof is harder than for Stewart's sequence). Still, Stewart's piece is more an artificial construction to motivate the cube-free sequence than an actual piece on chess proper. (Note that Stewart even notes that his rule for repetition is distinct from the actual rule, which refers to positions and not moves). – Steven Stadnicki Nov 21 '13 at 7:23
  • @StevenStadnicki, I agree with what you wrote. Also, thanks for the clarification (on positions and moves). – Joel Reyes Noche Nov 21 '13 at 7:27
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There is a cap on the length of a game of chess in terms of the number of moves. That's because of the Fifty-Move Rule. Any attempt to draw out a game indefinitely would trigger the fifty-move rule and result in a draw. The reason for this is simple. To carry on the game indefinitely, you have to:

  1. Move pawns an infinite number of times: impossible. The number of possible pawn moves is obviously finite as pawns cannot move backward.
  2. At some point, cease capturing your opponent's figures (otherwise you will capture their king, i.e. checkmate them, ending the game): this will trigger the 50-move rule.

Also, I suggest moving this to Chess.SE.

  • Under the fifty-move rule, the game is not a draw if both players don't claim the draw. Thus, a non-ending game of chess is possible. – Joel Reyes Noche Nov 21 '13 at 7:17
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The other answers have relied on the 50 move rule, and have pointed out the possibility of the game not ending if neither player invokes it.

As it is highly unlikely somebody would want to play a game of chess for thousands of moves in a regular game, it follows that such a game would be contrived solely for the purpose of playing out the longest chess game possible. Furthermore, as nobody would want to spend their entire life playing a game of chess just to hold the record for longest game of chess, this all would be a purely mental exercise.

However, given these constraints and the fact that an unending game of chess is possible if neither player claims a draw from the 50 move rule, it is still unsatisfying to say that a game of chess can go on forever. Since we cannot substitute chess players, eventually one or the other of the players will die of old age or some other cause and will be unable to continue thereby forfeiting the game or at least bringing it to an end. We can therefore calculate an upper limit on the number of moves that can be played before this occurs.

Assuming both players learn to play chess earlier than any person yet, say at 3 years old, and live to be older than the oldest person alive, say 120 years old, and that they play every waking moment, say 16 hours per day on average, and play speed chess averaging one move per second, and take only leap days off to rest, this yields an upper bound of 1 move/second * 86400 seconds/day * 365 days/year * 117 years or 3,689,712,000 moves as the longest chess game possible between two people when neither invokes the 50 move rule to claim a draw.

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The answer depends on preference:

  • If you apply a 50-move limit, the longest possible chess game is 5898.5 moves long.
  • If you apply a 75-move limit, it is 8848.5 moves long.

See https://wismuth.com/chess/longest-game.html for a detailed demonstration.

If you don't apply either of these, the next hurdle is draw by repetition (at 3 or 5 occurrences). I don't know if anyone has explored this systematically: perhaps a project for someone?

If you reject draw by repetition too, then you can carry on forever. Have a look at https://wismuth.com/chess/statistics-games.html#perft-ratios which argues that the maximum eigenvalue of chess (which will dominate the growth rate in the long run) is about 84.3.

Which approach is right?

  • If you are a player, then I guess you might say that the 50-move rule involves player choice, while the 75-move rule is mandatory, so go for the latter.
  • If you are a problemist, you might say that the 50-move rule only applies by default to retro problems. However you would probably be interested to waive that convention for this interesting problem. I think the don't know any problemists who are adopting the 75-move rule yet, and it's excluded from scope of conventions.
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Theoretically it can be infinite I think, but practically they say it's a draw when both players have only a King and a Knight left for example.

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    If we discard rules for draws, the game is no longer "chess". You didn't answer the question. – Lord_Farin Nov 21 '13 at 8:14

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