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In Go, there is a potentially repetitive sequence of moves called a "ko." So if your opponent takes a ko from you, you have to make a move elsewhere on the board, called a ko threat, before "taking back" the ko, in order to break up the repetition. But suppose someone "repeats" by taking back a ko, without making a ko threat. What are the penalties?

My understanding is that in a tournament, the penalty will be a forfeit of the game. But I've never seen this rule enforced in informal play. The standard "fix" is that the offender is forced to take back his ko "recapture," and make another move.

Is my understanding of the tournament rule correct? And is there an informal "house rule" that prevents the application of this rule in casual games?

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  • One of the situations where online go is nice. Can't make that mistake :)
    – CodesInChaos
    Jul 21, 2013 at 18:38
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    @CodesInChaos Even online you still can make that mistake, you just can't click. After trying to click several times, you will realize it: "oh, why can't I play there? oups!" It's just that your opponent doesn't know about it... Actually, I would prefer an online server where you can make such a move, and if you do, you lose immediately.
    – Djaian
    Jul 22, 2013 at 19:52
  • @Djaian At least on KGS the point(s) blocked by ko are marked with a rectangle.
    – CodesInChaos
    Jul 22, 2013 at 20:04
  • @CodesInChaos I know. But when you are thinking, you might still want to play there, and realize your mistake only when you click.
    – Djaian
    Jul 22, 2013 at 20:06

4 Answers 4

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In all official rules that I know this means instant forfeit of the game. Of course, in casual play, mistakes like this can be allowed to take back at the will of the players.

In tournaments I would strongly advise against making such a mistake as well as trying to ask for a take back. If the other player does not seem to allow a take back by his own active will, just resign and promise yourself to not make the same mistake again. Mental strength is part of the game, too, after all (just like time management, i.e. losing on time).

The reason players sometime forget to make a threat is often that after player A took the ko, player B takes some time to contemplate the threats, the board, counts the score, and finally simply recaptures the ko - the need to play a threat slipped out of their mind in the meantime.

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  • Apparently this isn't true for the official EGF (taken back if noticed within three moves) and AGA rules (taken back, treated as a pass).
    – CodesInChaos
    Feb 28, 2015 at 19:32
  • I was not aware of that. I thought the rules we used at the EGC (and other tournaments) are official, and did not cover this - or did they? I may have misremembered.
    – mafu
    Feb 28, 2015 at 21:55
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At least in European tournaments current rule is that if you play an illegal move you have to take it back, and play a legal move. You don't lose the game by default.

Illegal move

If a player makes an illegal move, and if this is noticed within three moves, then the game should be unwound to the move just before the illegal move, and continued. The referee may allow an adjustment of the time.

EGF general tournament rules

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  • This is reasonable!
    – Tomas
    Nov 11, 2014 at 14:49
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    AGA rules treat it as a pass: "Illegal Moves: An illegal move is one violating the rules. If a player makes an illegal move, it shall be taken back, treated as a pass, and a pass stone exchanged."
    – CodesInChaos
    Feb 28, 2015 at 19:33
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    @CodesInChaos: Since 2danplayer seems to be inactive, perhaps we should add the AGA rules and any other rules to this answer. After all, an ideal answer should cover all sets of rules. Sensei’s Library has an article HowIllegalMovesAreHandled covering AGA, Japan and China.
    – PJTraill
    Jan 20, 2018 at 12:55
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One important point to understand is that in an amateur tournament, whenever both players agree on something, then it's ok. For example, if one player plays an illegal move (like taking back a ko immediately) and the other says "take back your move and let's pretend nothing happened", then so it is: nothing happened.

Problems only occur when the players disagree. According to the rule, playing an illegal move is an immediate loss. That means the other player can, if he wants, declare victory.

Usually, weaker players will readily let their opponent take their move back. When it comes to stronger player, they tend to claim a win if the opponent makes such a mistake.

The worst that can happen is that someone else, not playing but just watching, interferes. As a tournament organizer I hate when people interfere in other people's games. That's usually when the fuss starts. The player agreed to let his opponent take back, but someone watching tells he should claim victory, and so on.

In a professional tournament the player making the mistake will resign as soon as he sees it (such mistakes are very rare in professional tournaments, but may happen sometimes).

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  • Roughly how strong do you think players usually are when they start resigning/expecting the opponent to resign?
    – PJTraill
    Jan 20, 2018 at 12:45
  • @PJTraill I have seen 6 dan players acting like ***holes during tournaments just because they see an opportunity to grab a win where they should lose (pretending the opponent is cheating while adjusting the clock for byo-yomi, continue playing a lot of tricky moves because they have more time, trying to disturb the opponent in any possible way, and so on). As a former tournament organizer myself, most problems occured with dan level players. Granted, technically speaking you can claim you won if your opponent make such a mistake. Kyu players tend to enjoy playing the game more than winning.
    – Djaian
    Jan 22, 2018 at 6:53
  • Amateur dan players, evidently, and presumably only some bad eggs — but sad anyway. As for •••holes, perhaps one might say trumpholes?
    – PJTraill
    Jan 22, 2018 at 13:59
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I have seen what you're describing in tournaments, too. A player made a mistake and took the ko when it was illegal, and the other player just made him take the move back.

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    This doesn’t answer the question:(
    – PJTraill
    Jun 11, 2016 at 11:38

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