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A quote from Wikipedia:

The objective is to 'checkmate' the opponent's king by placing it under an inescapable threat of capture.

Suppose two players (A and B) play the game using the old fashioned way (no computer involved). If player A has player B's king checked from two positions. Player B only knows how to block one of those two attacked positions. But there is a way for Player B to make a single move and block both attackers. What would be the proceeding of the game next? Should player A show player B how to do it? Or does the game end with player A winning even through this is not a checkmate?

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    If the king is placed in double check, then there is no way to block both checks and therefore the king has to move. If the king has nowhere to go, then it is check mate. – Dag Oskar Madsen Oct 19 '15 at 14:00
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    I have edited the title to make it more clear what the point of the question is. – dfan Oct 19 '15 at 16:48
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If player B is checked in two different ways, then there is by definition no way for player B to block both attackers with a single move. The only way to get out of double check is to move the king.

If there's a square for player B's king to move to that is safe, then he can get out of the check. If player B simply doesn't see it, then in a friendly game player A can certainly show player B where to move his king. In a competitive game player A should probably just let player B lose on time or forfeit by making an illegal move.

If there's no square for player B's king to move to that is safe, then it is in fact checkmate. Player A wins.

  • Would you like to try for en-passant capture cancelling both legs of a double-check? – Joshua Oct 19 '15 at 23:13
  • @Joshua How would that look like? Can you post a position where en passant comes immediately after a double check? I am fairly certain that you cannot double check with a pawn anyways, except for en passant. And if the last move was en passant, the next one cannot be en passant by definition. – phihag Oct 19 '15 at 23:17
  • Maybe it can't happen. I didn't seek far enough to find a complete solution. – Joshua Oct 19 '15 at 23:19
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    Update: no it can't happen. The en-passant capture and block is only possible if certain fairy chess pieces are in play. – Joshua Oct 20 '15 at 1:34
  • Not that it matters, but you can double-check with a pawn without en passant in the same scenario you could with en passant. From a new board, say you do the following: Pe2-e4, Pf7-f5, Pe4-xe5, Pe7-Ee, Ng1-h3, Qd8-e7, Ke1-e2, Pe5-e4, leading to this. Now, white can either do Pf2-f3 or Pf2-f4, followed by black doing Pe4-f3, double-checking white's king. Click here then "(animated) diagram" to see it in action. – MichaelS Oct 20 '15 at 4:08
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Each tournament has its rules, but what I was taught as a child when I played was that the agreement of the players was the end of the game; if you weren't checkmated, it was treated as though you resigned because you thought the position was a lost position. Shake on it, and the game is decided.

As the winning player, it was considered unsportsmanlike to not point out the available move, but not obligatory. The winner may not notice the move itself! I remember one game where castling was the difference between being mated and winning the game, but it never occurred to either player that, that late in the game, someone might not have moved either their king or rook. I also heard of one game which would have been won by an en-passent (but I never did confirm that it occurred).

In the end, it really depends on the situation. If there's more important things than winning (like having fun and learning), not pointing it out is bad form. If winning is all there is (which might happen in some master level play for large money), then... well... hopefully you don't make a mistake like this. Usually the mistake at the masters level is resigning from a winning position, and they usually don't need their opponent to tell them about the mistake later -- all of their friends and their chess computers will make sure they know afterwards.

  • "Each tournament has its rules" Actually, the rules are usually set by the controlling chess federation. "If winning is all there is (which might happen in some master level play for large money)" In any tournament, you don't discuss the game with your opponent. That includes telling them whether or not they have any legal moves. – David Richerby Oct 19 '15 at 20:46
  • @DavidRicherby Then that would tell me that you play mostly in tournaments where winning is everything, right? – Cort Ammon Oct 19 '15 at 21:20
  • You cannot escape from check by castling though, nor can the king cross an attacked field in it! Hence I assume the forgetting of the possibility to castle was a few moves before the end of the game ...? That would not seem like directly comparable to the OP situation – Hagen von Eitzen Oct 19 '15 at 21:35
  • @HagenvonEitzen It was a situation where someone resigned because they saw no way out of a forced mate over several moves which was not actually forced because castling was an option. Its a little different, given that the player resigned, rather than believing the board position was checkmate, but it seemed relevant enough to be a point of reference. – Cort Ammon Oct 19 '15 at 21:38
  • @CortAmmon I don't know what you mean by "play mostly in tournaments where winning is everything." In any rated tournament (which very definitely is not restricting to situations "which might happen in some master level play for large money"), it is forbidden to discuss a game in progress with anyone, including one's opponent. You seem to equate this with some sort of despicable win-at-all-costs attitude but it's a simple matter of the rules of tournament play. If you want to discuss the game afterwards, fine. During the game, no. – David Richerby Oct 19 '15 at 21:45
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I guess the question is what to do if one player doesn't find a legal move, but it is not checkmate.

In a tournament game he would lose on time, lose because of repeated illegal moves or possibly resign.

In a friendly game between amateurs you can handle the situation any way you like. It depends on whether you value the competitive or the social side of playing chess more. I would say if you allow takebacks, you might as well show player B the defence, otherwise a loss is the sensible result.

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Once the players agree that the game is over and agree who has won, it's over. If later the player who lost realizes they had a saving resource, it is too late. The game is over.

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