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I've read about "mate prevails" here on this site, but I couldn't get a clear view on one odd point. I get that the mating move prevails, but if you couldn't do it before your flag falls, you lose.

Let's say it's my opponent's turn. I'm about to mate in 1. They hit the clock, and I run out of time. Then he notices it and claims it while I'm performing checkmate, at the exact same time.

Under FIDE rules, what happens?

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Theoretically the situation is very simple, whichever happens first stands. In practice it is often difficult to determine what happened first though.

If you insist that both the flag falls and the mating move is made at exactly the same time (the same nanosecond say) then my answer would be that this can never happen. It is as in statistics - the probability of a single point is zero, i.e. two events cannot happen at the same time.

As an arbiter you need to decide according to what you believe happened first. How to determine that is an old question (with no clear answer obviously) but that is not what your question is about if I understand it correctly .

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  • I mean I understand that it never happens exactly at the same time but ... I can see how it could get a little tense if that happened pretty much at the same time.
    – Sebastien FERRAND
    Nov 27, 2015 at 6:01
  • @SebastienFERRAND Yes, this makes it difficult in practice. But it is a more general question of "How does the arbiter find out what happened when they did not see it?". There are many such situations and the guiding principle is that the arbiter has to do their best to find out and to make a judgement in the best interest of the competition. The arbiter may use all possible sources of information for instance they can ask the spectators what happened. In this specific situation if they cannot really find out giving the benefit of doubt to the mating player sounds reasonable to me.
    – IA Petr Harasimovic
    Nov 27, 2015 at 18:12
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If no arbiter saw this happening and there is a dispute, then the player who claims the flag fall can't prove that it happened before the move. The player who checkmated the opponent can show that he did (the position is on the board). So the checkmate will win.

If an arbiter saw what happened, a solution is probably possible, after all the claim of a win on time (or the arbiter announcing same) will happen slightly after the flag fall. If the claim and the checkmate occur at the same time, then the flag fell first and he lost on time. If the flag fall and the checkmate occur at the same time, then the claim was late and the checkmate stands (the game is already over, so claims are irrelevant).

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  • A technical comment, a flag is considered to have fallen when the arbiter observes the fact. The arbiter does not have to announce it. If they are certain (meaning they saw it happen) the flag has fallen before the mating move was completed they will declare the game lost on time for that player. In practice they may not be so certain very often though.
    – IA Petr Harasimovic
    Nov 27, 2015 at 18:30
  • @IAPetrHarasimovic "A technical comment, a flag is considered to have fallen when the arbiter observes the fact". Not strictly true. According to 6.8 "A flag is considered to have fallen when the arbiter observes the fact or when either player has made a valid claim to that effect. " If what you said were true then players playing in competitions where no arbiter is present could not lose on time, a farcical situation.
    – Brian Towers ♦
    Jan 20, 2016 at 16:00
  • @BrianTowers Of course I did not mean only if but that is quite obvious (after all the rest of my comment explains what I meant).
    – IA Petr Harasimovic
    Jan 20, 2016 at 17:14

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