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I was playing a game with a friend online. A situation came up where he put me in check with his bishop, and I moved my rook to block it. His king was on the g file and my rook was on g2. The website said he was in check. 

[fen "6k1/8/2b5/8/8/8/6R1/7K b - - 0 1"]

Position after Rg2+ , showing only the pieces important to the question.

I didn't think it could be a check, since legally, I could not attack his king or move my rook because it would put my king in check. Can a piece put a king in check even though moving that piece would be an illegal move?

This is odd, because since he was forced to respond to the check, I was able to make a couple moves and leave him with only a couple of blocked pawns, while I was able to promote a pawn and mate in a couple moves. I won, but it feels like cheating. He had a better position and should have won.

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    By the way, beating a better position is not an indicator of cheating. :)
    – Daniel
    Commented May 4, 2013 at 22:35
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    You might want to think of it that way: To win a chess game, you have to take the enemy's king and the rules restricting your moves when you are in check are just here to prevent you from doing a move that would let the enemy take your king on his next turn. But if you take his king before he gets to play his next turn, then you win.
    – xavierm02
    Commented May 4, 2013 at 23:45
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    Couldn't your opponent just take your rook with the bishop for material advantage and check?
    – justhalf
    Commented Jul 1, 2019 at 23:47
  • @xavierm02 nice comment
    – ramanujan
    Commented Apr 3, 2023 at 10:57
  • @justhalf How would it be material advantage since the king can immediately capture the bishop for a draw?
    – Michael
    Commented Jun 10 at 18:38

3 Answers 3

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Yes. Besides making sense, it's also explicitly stated in the rules of the game:

3.9 The king is said to be 'in check' if it is attacked by one or more of the opponent's pieces, even if such pieces are constrained from moving to that square because they would then leave or place their own king in check. No piece can be moved that will either expose the king of the same colour to check or leave that king in check.

Source: Laws of Chess on the FIDE site

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Can a piece put a king in check even though moving that piece would be an illegal move?

Yes.

One way to see why this makes sense is to imagine "checkless chess", a game that is just like chess, but you win by capturing the king, not by delivering checkmate, and no one is forced to move out of check. This game is exactly the same as chess except that it ends one move later and stalemate is generally a loss for the side that would have to "move into check".

In checkless chess, he moved Bc6, then you moved Rg2, and if he makes some other random move, you can win with Rg2 x Kg8 before he wins with Bc6 x Kh1. So your check really is more "important" than his pin.

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    In a real "with check" game, the definition of "check" is that the king is attacked by a piece, not that the piece could legally move to the square that the king is on. My "checkless chess" example was intended to illustrate why this definition makes sense.
    – dfan
    Commented May 4, 2013 at 19:11
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    Thank you very much! I have only been playing a short while and still have much to learn.
    – ken
    Commented May 4, 2013 at 19:14
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    +1 Basically, the point is that your theoretical capture of his king would end the game before (just before) he is able to capture yours. So if he was allowed to leave his King in check by your pinned rook, you would win.
    – Daniel
    Commented May 4, 2013 at 22:11
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    You don't need to imagine "checkless chess" as that's how every high speed game I've played has been handled--check means nothing, the game ends with the capture of the king. It's crazy when everyone watching sees a check that neither player notices for multiple moves!
    – Loren Pechtel
    Commented May 4, 2013 at 23:37
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    @AricFowler I did not know that the term "checkless chess" was already used with a different meaning. Feel free to substitute some other term such as "chess without check" everywhere I say "checkless chess".
    – dfan
    Commented Aug 10, 2016 at 20:27
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A piece can give check even when it is pinned. This is the main "exception" to the rule that a pinned piece cannot move.

The reason is, your pinned piece giving check "takes" the opposing king first. (In this case, it's your rook at g2 on the g file.) That's BEFORE his bishop would take your king.

So by the laws of the game, your friend had to move his king, you made your moves, and won. All perfectly legal (and tactical). Good work.

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    Notice that it is no exception, because a piece need not "move" to give check. Check is by definition the occupancy of a square of the board with the potential action of the piece and it illegal in the first place for the opponent's king to be placed on such square.
    – gented
    Commented Dec 17, 2016 at 20:52

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