Capturing a piece or moving a pawn irreversibly changes the state of the game and thus resets the counter for the 50-move rule. Since castling irreversible changes the state of the game too, it seems to me that it should reset the move counter too, but it does not. Why is that?

  • Note that castling can be done manually over several moves. A one-move castling changes the "state" of the game similar to a multiple-move castling would. It simply changes the positions of some pieces, just like normal piece-shuffling would. – Scounged Oct 12 '18 at 17:19
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    I can think of several cases where that is not the case. Some positions can be achieved only by one-move castling. – koryakinp Oct 12 '18 at 17:30
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    How contrived are the examples you're thinking of? I am sure that there exist positions that can be reached only through castling, but I cannot think of a single one off the top of my head. Moreover, it can still be seen as a more "advanced" form of piece-shuffling. – Scounged Oct 12 '18 at 17:38
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    @Scounged A position where the king has castled and none of the pawns have moved would be one I could think of off the top of my head where manual castling could not have happened - but yeah, that's rather contrived, as the enemy knight must have taken the bishop on its original square without the king retaking. – D M Oct 12 '18 at 21:11
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    @koryakinp I suspect you're not really asking "Does castling reset 50 moves rule?" since you state "it does not" later in the question. Perhaps you should edit the title to better reflect what you're really asking? – D M Oct 12 '18 at 21:36

I don't know the reason, but I can make a guess.

In standard algebraic notation, it's fairly easy to look at a scoresheet and see whether a capture has been made or a pawn moved. Captures have an "x" in them, and pawn moves don't have a piece listed. But if loss of castling rights also triggered the 50 move rule, you'd have to look at the rest of the game (instead of just the last 50 moves) to see whether castling rights had previously been lost. This would seem to increase the burden on arbiters.

Also, it's rather unusual for this to matter. Most people are going to move their king (and/or both rooks) before they reach a point in the game where the 50 move rule would begin to apply.

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    Furthermore, if we follow the logic of the question, we should take moving the king moving for the first time, when the rook has not moved, to reset the counter. – Acccumulation Oct 13 '18 at 4:08
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    I do not see why it is so difficult to detect rook moves on scoresheet. It is way harder to detect threefold repetition. – koryakinp Oct 13 '18 at 21:47
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    @koryakinp It's not terribly difficult, but it would make it slightly more difficult, and since it almost never comes up there wouldn't be much benefit. Sure, you can see rook moves, but it may not be easy to know which rook. Threefold repetition is sometimes more difficult to detect (especially if it's not in consecutive moves) but there's not much you can do about that. – D M Oct 14 '18 at 3:17
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    This is no valid reason. Of course, as an arbiter, i’m glad that i can test the score sheet in an easy way. But 0-0 or 0-0-0 for castling is also easily detected. A non capturing move of a certain piece type might be more difficult to detect, especially if the notation is in a language that i do not speak. But „more difficult“ still is not „difficult“ for an experienced arbiter. – Christian H. Kuhn Sep 26 '20 at 20:51

After some discussions in the comment section I think it's time for me to start answering this question to the best of my understanding. First off, let's discuss the use of the concept "the state of the game". Although I sort of understand what you're trying to say I still wonder about weird cases such as the following:

  1. e4, e6 2. e5, d5 3. Nf3

Has the "state" of the game changed on move 3, since white was allowed to capture the d5 pawn en passant before, but is now unable to do so after the third move? According to you, this would mean that the 50 move rule counter should be reset not on 2...d5 but instead on 3. Nf3. Since you only give examples of what would change the "state" of the game rather than a definition of what a state-changing move should constitute explicitly it is difficult to know exactly what you're trying to convey when using this concept. Please feel free to comment what you think should happen in the above scenario.

Let's go on to talk about the reasoning behind a rule such as the 50 move rule. What is the purpose behind it? The purpose is to force players who refuse to accept a draw to demonstrate an actual plan of action for winning the game. This is done by forcing the stubborn player to eventually try to either

a) resolve the situation on the board somehow by capturing/trading material, or

b) to make some progress in the position by pushing a pawn forward, presumably trying to queen it.

If none of actions a) or b) are taken within 50 moves, then it's determined that neither of the players is making an honest attempt at winning the game, and it's up to either player's discretion to make a draw claim when the move limit has been reached. Nowadays it is possible to show with the help of tablebases that there are indeed positions where one side can force a checkmate but has to shuffle pieces around for way more than just 50 moves before either of actions a) or b) can be taken, however the rule still stands. This is because of the fact that positions where one side can force a win only if they're given more than 50 moves of piece shuffling are extremely rare, and in practice impossible to determine for human arbiters.

With the above motivation of the 50 move rule in place, what is the case for amending the 50 move rule to include castling as a move counter-resetting move? All in all, it doesn't really try to resolve any situation on the board by making a capture, and it does not try to make some sort of progress either by moving a pawn forward. The only thing castling really does is to shuffle two pieces along the first rank. Sure, after castling once you may not castle again, but what does that have to do with attempting to make progress of some sort in a game of chess in contrast to other moves made by other light/heavy pieces?

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    The state of the game changes with every move. In the context of 50 Moves Rule, it is important not just the change of the state of the game, but irreversible change of the state of the game. I see no reasons why certain actions, which lead to irreversible change of the state of the game reset 50 moves counter, but others - do not. It would much elegant to reset the counter after every such action. Since en-passant capture can be done directly only the pawn move it is irrelevant to 50 Moves Rule. – koryakinp Oct 13 '18 at 23:27
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    @koryakinp The reason is that certain moves are seen as indicators of trying to resolve situations/making progress, while others are not. Let us consider another weird example inspired by a comment made by Acccumulation: 1.a4, e6 2.h4, d6 3.Rh3, Be7 4.Ra3. Should the move counter be reset after white's 4th move? Based on your previous reasoning it would be surprising if you don't think so, since the right to castle is permanently and irreversibly gone for white after move 4. – Scounged Oct 13 '18 at 23:56
  • Sure, I believe it would be reasonable to reset the counter after every irreversible change. In this case yes, white lost their right to castle. – koryakinp Oct 14 '18 at 0:36
  • @koryakinp Castling is NO irreversible change. You can „uncastle“ by two moves of the king and a move of the rook. And after that, you can „recastle“ by three moves. The difference is only the absence of the abbreviation (and that’s why castling rights are relevant for the 3fold repetition of position). – Christian H. Kuhn Sep 26 '20 at 20:47
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    @koryakinp e.p. is also irrelevant for 50 moves. e.p. right exists after a pawn move - which resets the 50-move counter. Taking e.p. is a pawn move AND captures a piece - which resets the 50-move counter. Not taking e.p. changes nothing. Ok, the e.p. right disappears. But the pawn CAN be removed later on by another move. So nothing irreversible has happened - if you did not move a pawn or capture a piece instead, of course. – Christian H. Kuhn Sep 26 '20 at 20:48

Capturing and move pawns are not reversible, in the sense that the position occurring before the capture or the pawn move cannot any longer occur in that game.

To make an example, after 1 e4 d5 2 e:d5 there's no way back: white will never have a pawn in e2 again so Black's with the d7 pawn. Also, Black will never have 8 pawns again.

Castling in not irreversible in this sense. After, say, white castling king side it is not unconcievable that the King may return in e1 and the rook in h1 with the position prior to castling occuring again on the board.

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    The point of the question is that if White castles kingside, he will never again have Ke1, Rh1 and castling rights. So, in a way, the transformation of the position is not reversible. – Evargalo Oct 15 '18 at 15:18

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