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I was watching the match between Ian Nepomniachtchi vs Hikaru Nakamura and Nakamura used both hands to castle.

Why is it illegal to castle using both hands? What's wrong with that?

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    "What's wrong with that?" Nothing really. One could play chess with two hands, probably, but it would be a slightly different game then.
    – Trilarion
    Aug 22 '18 at 8:40
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    Very simple: you must play and press the clock with the same hand to ensure you are completing your move before pressing the clock. If you use two hands to play, you are breaking this rule. Aug 23 '18 at 17:44
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    I don't have enough rep to answer but there's a touch-move rule in Chess. If you moved with two hands it would be unclear which piece you touched first. If you touch your Rook first you would have to move your Rook and not be able to castle. Only if you touch your King first and move it two spaces can you castle. Furthermore, once two hands get involved it's not clear that the clock has been hit after the move has been made. I had this at the end of a game when an opponent, short on time, moved with one hand and hit the clock with the other. I immediately hit the clock back to his side... Aug 23 '18 at 19:55
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    ...and told him to depress it with the hand with which he moved. Aug 23 '18 at 19:56
  • @CJDennis I need 10 rep excluding association bonus to answer. I only have 1. Aug 24 '18 at 8:08

12 Answers 12

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The official procedure to castle is (Schiller 2003:19–20 from Wikipedia):

first move the king with one hand and then move the rook with the same hand.

By using both hands the player can save time, as would by as using different hands for moving (like promoting a pawn to queen) or moving with a hand and hitting the clock with the other.

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    Very good answer, it appears from your answer that it is about timing and not which hand hits the timer.
    – Malachi
    Aug 21 '18 at 15:06
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    I'd wager this is for the reason Mawg gave. Moving the king first takes away any option to decide against castling mid-move based on your opponent's reaction. Once the king is released, the only legal move left is to castle. Aug 21 '18 at 16:43
  • @LordFarquaad that is indeed a very good reason, but does not explain why you can't do any other moves with both hands (like turning a pawn into a queen) or moving with a hand while the other one is at the clock.
    – Bernat
    Aug 22 '18 at 6:58
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    When competing at world class level every millisecond counts. If over the course of a game one competitor gains half a second of thinking time over the other - in any other sport there'd be a stewards inquiry in how that happened. Aug 22 '18 at 14:01
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    Also the risk of ambiguity concerning which piece was touched first, and furthermore that all the pieces have stopped moving before the clock is pressed. E.g. one might otherwise touch the rook first, before realising an error and then grabbing the king with the other hand to change the move into castling. Aug 24 '18 at 9:56
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Disclaimer: this is probably not the answer, but it makes for interesting thinking.

This reminds me of a string bet in poker. String bets are illegal, because they can be used to gain information – “I’ll see your $5”… < watches opponent’s face> "… and raise you $20”.

By moving the king first, you are making what would otherwise be an illegal move (moving the king by more than one square) and cannot “string bet”.

If you were to move the rook first, you might observe your opponent’s face, then quickly choose to move the king, claiming that you had intended to castle all along.

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    Welcome to the Chess SE site! You're right in your disclaimer: this would be better posted as a comment on the OP since you are theorising instead of providing a conclusive answer.
    – Aric
    Aug 21 '18 at 14:16
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    You're answering the question of why you can't move the rook first, but that's not the question that was asked. And incidentally, it's perfectly legal to pick up your king and hover it like you're going to castle kingside, and then change your mind and castle queenside, so long as you don't let go of the piece.
    – D M
    Aug 21 '18 at 17:50
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    That "which side am I going to castle to?" trick sounds like a good string bet/fishing for information (+1) Aug 21 '18 at 18:08
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    This has no value in answering the question. Also, it is not true, as you can pick up and move any piece while observing your oppenent's face, as long as you do not leave the piece in an illegal position.
    – J. Win.
    Aug 21 '18 at 20:23
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    Opponent-reading doesn't matter in chess, though, since you generally have more to gain from reading the board than from reading your opponent's mind (full information game).
    – Brilliand
    Aug 21 '18 at 21:59
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I know in general the rules say the hand you move your piece with has to be the one that hits the clock. If you castled with two hands which one would you use to hit the clock? This might be part of the reason for the rule saying you can only castle with 1 hand.

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    I really don't think picking a hand to push a button is that big an issue. We sent men to the moon, I think we can handle this.
    – Alexander
    Aug 22 '18 at 1:22
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    @Alexander It's more a question of fairness to your opponent. The rule exists to get your hand off the board before hitting the clock so your opponent can make the next move. If you're still moving a piece with your left hand while you hit the clock with your right, you cut into your opponent's time. But the rule is pretty much impossible to follow if you use both hands. Aug 22 '18 at 4:40
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    On the other hand, your opponent would know what is happening before you put the second piece down.
    – PStag
    Aug 22 '18 at 5:46
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    @PStag this assumes it is a legal move. But part of the opponents move is to actually check that. And she can't do that until the pieces are put down. Which might not have happened when the clock is hit when you are using both hands. Aug 22 '18 at 6:01
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    @Meni That rule would be harder to objectively enforce. Aug 22 '18 at 16:18
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Chess rules are all about sequence. If you use two hands, you don't know which happened first. So you always use one hand. First, you make your move, then you hit the clock.

As others have pointed out, the rook move alone could be one or two moves. Maybe, you were just moving your rook, maybe you were castling. The King move, however, does determine the one and only move. There is no other King move that allows the King to move two squares. By moving the King first, then moving the rook, there are no questions as to the real move. The Rook, while being part of the move, is forced. Because this is a sequence, you have to use one hand to play it.

As a side note, if you could use both hands, and you were playing with a clock, you could move the rook and hit the clock with one hand, and move the king with the other hand, and then there would be a sequence argument whether the king had moved before or after the clock hit, and thus whether it was a rook move or a castle. By requiring one hand to do all three (king, then rook, then clock), there is no question as to what went on. The king moved two squares first. That defined the move. Then the rook moved (with the same hand), then the clock was hit after the board was in a valid situation. No arguments.

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    +1, I think this is the main reason. One second more or less, once at most, usually doesn't lead to heated rules disputes - but determining whether rook or king was touched first very much does (when using both hands, most people tend to touch the pieces almost simultaneously). This rule exists to make life easier for arbiters.
    – Annatar
    Aug 22 '18 at 7:20
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why is it illegal to castle with both hands?

Because the rules say so.
From the FIDE Laws of Chess -

7.5.4 If a player uses two hands to make a single move (for example in case of castling, capturing or promotion) and pressed the clock, it shall be considered and penalized as if an illegal move.

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There is a very important rule for castling with one hand. In the Fide Handbook it is rule 4.1.

4.1 Each move must be made with one hand only.

The rule applies for all parts of the move, including capturing, castling and promoting.

Now for the reasoning, of why a move needs to be executed with exactly one hand: In case that you are making an invalid move there are specific rules on how to proceed after taking back the invalid move. As a general rule you have to move the first piece that you touched (rule 4.3.a). In case that you use both of your hands for castling, it would be pure luck to say that you need to do another king or rook move. Instead the order is clear. You first touch the king (and move it two squares). Then you touch the rook and move it to the other side of the king.

And why do you have to move the king first? Again it is to reduce any ambiguity. If you'd move the rook next to the king (without touching the king) you already completed a legal move. Castling now would be very weird as for the only time in chess a completed move could still be extended. Your opponent therefore could already make his move and you could still go: "I'm not finished yet, please take back your move". (In an extreme case you could wait for all the time on your clock before announcing that you are castling).

For these reasons it is important that you execute the castling in two steps (first king, then rook). The additional rule that you can only use one hand is not of practical importance as long as both parts of the move are clearly distinguishable.

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    pure speculation: (1) "one hand only" minimizes time benefits over handicapped/injured players who can't use 2 hands (2) "one hand only" is a remarkably simple rule, and simple rules are easier to teach, follow and arbitrate. Aug 23 '18 at 7:39
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It would permit a rather interesting ambiguity on the board.

Many have mentioned that the king has to move 2 spaces first, so that there's only one possible valid thing to do. So let's assume we do this.

With one hand, move the king two spaces. Once the king is in place (proving you intend to castle), the other hand grabs the rook and moves it. Meanwhile, the first hand goes to hit the clock.

It is possible that the first hand hits the clock before the rook is moved. Now you have a really funny situation where your hands are still moving pieces while it is the opponent's turn! Needless to say, this could be a big deal, especially if the winning move is to take that rook.

To avoid this case, one needs rules which ensure that all pieces are in their final position before the clock is pressed. Whatever rule FIDE chose, this is the key requirement placed on that rule. Hands off the board when it's not your turn!

If we use pi caluclus we can define a process where the clock press must occur after both moves are complete, regardless of which hand does what. So in theory, one might define a different rule which permits two-handed operation. However, in real life practical situations, such definitions are unreasonable. They work great with transistors and wires, and less well with human hands. Instead, we simplify the process by not allowing things to happen in parallel.

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In chess, whenever you touch a piece, you MUST move that piece. If you were to begin castling using two hands, and then removed your hands, it would be unclear which piece you would be required to move. This is why it must be done with one hand.

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    If you began castling using two hands, and removed your hands, it's quite clear that you intended to castle -- it's the only permissible two-piece move.
    – Mark
    Aug 21 '18 at 21:29
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    @Mark that only holds true if you didn't attempt to make an invalid castling move. If the castling is invalid, you still need to decide which piece was touched first. Aug 22 '18 at 8:46
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    @Mark That doesn't matter. Even playing one handed, I could touch my king and then my rook (or vice-versa). You could argue that touching both of those pieces clearly indicates that I want to castle, but that is not what the FIDE rules say: the FIDE rules say that I must move the piece I touched first if I can legally do that. So I'm allowed to touch my king, then touch my rook, then play Ke2. Or touch my rook, then touch my king, then play Rg1. But if I touch my king and rook with different hands at almost the same time, and decide not to castle, can I play Ke2 or Rg1? Aug 23 '18 at 13:55
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It's because you can't save yourself time by moving with more than one hand like that. Imagine one minutes where people did that. Castle would be over powered for time management reasons. Therefore, you castle with one hand, and the problem is avoided.

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    I don't understand what you mean by " Imagine one minutes where people did that. Castle would be over powered for time management reasons." Aug 21 '18 at 21:56
  • @DavidRicherby if you play in bullet and you're short on time, but have had king safety for some strange reason, you could castle with two hands very quickly. If you use one hand castling likely costs a second. If you use two maybe it costs less. That would give people who could castle a distinct time advantage, my guess is a full second, over those who can't. I don't think that's the point of bullet chess since chess is largely skill based. Just my two cents. Additionally, having two ways to castle means that people who still castle with one hand are just at a disadvantage for a petty reason.
    – user14104
    Aug 21 '18 at 22:01
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    Oh, I see. Still seems pretty insignifcant, to me. And I definitely don't buy the idea that allowing people to castle two-handed should be illegal because it disadvantages people who want to castle one-handed. People who don't want to do the best thing the rules allow will always be disadvantaged. ("We should ban running in the 100m sprint because allowing both running and walking disadvantages competitors who want to walk.") Aug 21 '18 at 22:09
  • @Steve Is there even such a thing as OTB one minute bullet?
    – Annatar
    Aug 22 '18 at 7:12
  • @Annatar I don't know if there are formal competitions but surely there are people in the world who have chess clocks and chess boards and set the clock to one minute each. Aug 22 '18 at 10:44
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I think the reason is a side effect of only being allowed to use one hand to make moves and to hit the clock. Otherwise in mutual time pressure it would be easy to end up making multiple moves when one is only allowed to play one before the opponent gets a go. Multiple hands could more easily support intentional cheating or create situations that an arbiter would find difficult to unwind if summoned

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this is speculation, but I think it's because FIDE wants to reduce the difficulty of knowing whether the rules are followed. One must use the same hand to move the piece and hit the clock because FIDE wants people to not hit the clock before they finish their move, and if different hands were used for the two actions, it could be unclear which happened first. And if someone uses both of their hands for a move, then clearly at least one of the hands used in the move will be different from the one used to hit the clock, making it difficult to see whether the other hand completed its portion of the move before the clock was hit. And it's not just what is easy to see, but what is clearly distinct. If we're trying to decide which hand completed its motion first, that is a matter of perception. But if we're trying to decide whether two hands were used, that's a clear binary. It's easy for two players to have differing opinions as to which hand completely its motion first, but whether two hands were used is an objective fact, and if the players disagree, that strongly suggests that at least one of them is just straight up lying.

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Legal is relative. It's illegal in FIDE rules as you can see with other answers.

But actually it's legal in American rules, at least in USCF rules in 2015. See what Hikaru Nakamura said in 2015.

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