By number theory, if the 75-move rule has been reached, the 50-move rule will also be reached. This has no meaning. It is like saying "I would buy this book if it cost under $60 and it costs under $40". So, what is the logic behind that?

  • In cases where the 75-moves rule applies (e.g. 2B vs N, IIRC), it replaces the 50-moves rule : you cannot claim a draw after 50 moves, but you can do so after 75 moves. – Evargalo Oct 31 '18 at 13:44
  • So do you mean that the 50-move rule does not apply to 2B vs N games? – Wais Kamal Oct 31 '18 at 14:10
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    @Evargalo that used to be the case (in various forms) from 1928 to 1992: en.wikipedia.org/wiki/Fifty-move_rule#History – Glorfindel Oct 31 '18 at 15:22
  • Yes, I was confused between two different 75-moves rules. Annatar's answer is spot on. – Evargalo Oct 31 '18 at 16:23

From the FIDE Laws of Chess:

50-move rule:

9.3 The game is drawn, upon a correct claim by a player having the move, if: (...) the last 50 moves by each player have been completed without the movement of any pawn and without any capture.

75-move rule:

9.6 If one or both of the following occur(s) then the game is drawn: (...) any series of at least 75 moves have been made by each player without the movement of any pawn and without any capture.

(emphasis mine)

As you can see, the decisive difference is that the 50-move rule only applies if a player correctly claims it, while the 75-move rule applies independently of any claims. So, in the 50 to 75 move window, the players can claim for a draw, but don't have to (and play on). Only when the 75th move without pawn moves or captures has been reached, the arbiter may interfere and declare the game drawn.

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    The better question is, given that at 50 moves it is objectively correct for one of the players to claim a draw, so why not just make it forced then? – eyeballfrog Oct 31 '18 at 18:55
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    @eyeballfrog: perhaps because both players still believe they can win when their opponent makes a blunder, or runs out of time. If both players believe they can win then why would either declare a draw when they did not have to? – Eric Lippert Oct 31 '18 at 19:49
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    @eyeballfrog The 50 move rule requires a player to notice it. That's not a given. – Loren Pechtel Nov 1 '18 at 11:11
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    @EricLippert The same argument applies to the 75-move rule, which does cut the game off. This is poor rules design--either forcibly end the game or don't. – eyeballfrog Nov 1 '18 at 14:45
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    @eyeballfrog It is not always "objectively correct for one of the players to claim a draw". For instance, say Black and White are playing in a tournament; it's the last game for both of them, currently each has n points, and the tournament leader has n+1 points with no more games to play. The only way either Black or White can catch the leader is by achieving an outright win for a full point. In this case, claiming a draw is a lose-lose situation. Not all games are zero-sum. – Geoffrey Brent Nov 1 '18 at 20:38

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