Answers.Body: <p>I will need to make an educated guess as to what rules come closest to what you have described, and I think it is that:</p>
  
  <ol>
  <li>You are checkmated whenever your king is attacked and if it were your turn next, you would not have any moves that would result in your king not being attacked.</li>
  <li>An opponent causes checkmate on you, if at the start of their turn you are not checkmated, but at the end of their turn you are.</li>
  <li>If at the start of your own turn you are checkmated, then your king is removed from the game and the opponent who last caused checkmate on you will gain control of your pieces.</li>
  </ol>
  
  <p>Under these rules, merely piling on a check while it was already checkmate would not cause checkmate.</p>
  
  <p>The rest of this post is background and further analysis of checkmate, that informed my answer and may help to understand it:</p>
  
  <p>According to <a href="https://en.wikipedia.org/wiki/Checkmate#History" rel="nofollow">Wikipedia</a>, it was the Persians who abandoned king capture and introduced warning of check in order to not accidentally end the game.  So let's go back in time before the introduction of checkmate and reanalyse.</p>
  
  <p>In two-player chess checkmate is a situation where at the end of your turn all parties are certain that you will (be able to) capture the king on your very next turn (and in two-player chess there exist no reasons not to actually capture the king).  Because of this certainty of how the next two turns will play out (a doomed evasive move and a certain king capture), there is no point in actually playing those moves, so both can be (and have been in FIDE chess) eliminated from the game with no detriment to the game.  (Very much the same thing happens when a player forfeits the game upon realizing that the game is lost and there is no point playing any further.)</p>
  
  <p>But if there are three or more players a third party can intervene on behalf of a threatened king and (unless the game ends when the first king is captured) there may be more pressing matters than capturing an opponent's king left in check (like defending your own king).  These differences make it impossible to say with certainty when king capture is actually going to take place before it actually happens, thereby invalidating the reasoning which allowed us to eliminate king capture from the (end of the) game.</p>
  
  <p>It is of course possible to try and use a definition of checkmate that is equivalent to the full checkmate described above in two-player chess also in chess with three or more players.  Indeed checkmate is usually defined in an operational way that is equivalent to full checkmate in two-player chess: your king is attacked and you have no move that will result in your king not being attacked. </p>
  
  <p>When you try to use that definition in multi-player chess you will need to think about on which player's turns one can be checkmated.  If your king is attacked and you have no move that will result in your king not being attacked, because it is not even your turn, are you checkmated?  Apparently that was something you rejected, but then it becomes hard to say who checkmated you.  It is possible for example that the player before you causes your king to be attacked by moving one of their pieces out of the way of a third player's piece, and even causes you to not have any moves that result in your king not being attacked, even though they are not even checking you with any of their own pieces.  Have they checkmated you?  I think most people are inclined to say yes.  But if you want to know if they caused checkmate or not, then you need to be able to say whether or not you were already checkmated before their turn or not.  Thus you need a way to determine checkmate even when it is not your turn.</p>
  
  <p>One extreme is to say that every check from a player not immediately before you is checkmate (because as it is not even your turn you have no moves to remove that attack).  Another way is to pretend that it were your turn and see if you could do something.  I have recently even read rules for multi-player chess where it actually becomes your turn if you are checked, and the player whose turn it would have been had you not been checked continues after you (and you may or may not forfeit you actual next turn).</p>
  
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Answers.UserDisplayName: hkBst
Answers.Body: <p>I nearly agree with @hkBst, who says that one should determines the beneficiary at the beginning of the mated player's turn, by asking who would be able to actually capture the king (in the pre-Persian format). This would certainly give a unique result.</p>
  <p>The removal of the king may also cause discovered checkmates to happen to as many a 4 other players in the game, if the board were large enough. And when they are eliminated for each of them that could result in another 3 discovered checkmates happening. Thus a cascade of checkmates might happen and the original mater might never have to move again.</p>
  <p>Alternatively, the elimination of the king might create a discovered mate of the original mater himself! Does this mean that the original mate was illegal? I don't think so, in that the removal of the king in this case behaves like the mated player's move.</p>
  <p>I also can't resist pointing out that the questioner says that under their rules, the mated king is removed <strong>after</strong> that player's turn. This suggests that the player gets to make some &quot;last dying move&quot;, illegal in the sense that it doesn't avoid check, because it can't.</p>
  <p>This would allow a mated player in some circumstances to select which of the opponents would inherit all the pieces. In a multi-player game like Diplomacy, selecting which of one's attackers will be the beneficiary is a very real threat, which can discourage attack and keep small fry alive. This is perhaps the most apposite possible example of the chess saying that the threat is more powerful than its execution, in that this can only be triggered if the player has definitively lost.</p>
  <p>Can one capture pieces one has inherited, or are they now part of one's own force? If the latter, then it's possible that one might be stalemated as a result of inheriting pieces that one can no longer capture.</p>
  <p>Finally, how <em>do</em> stalemates work in this game? Are the pieces frozen? Can they re-awake later potentially? Does the stalemated player need to hang around in case they are brought back to life?</p>
  <p>So many questions about this brilliant mechanism...</p>
  
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Answers.UserDisplayName: Laska
Answers.Body: <p>Checkmate is when the King is in check and no move would be able to remove him from check.  If the Bishop check is a move that delivered checkmate, even if he was checkmated before, is the last checkmate and the pieces would be his.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2016-03-18T13:07:00.220Z
Answers.Text: How can you deliver checkmate when it is already checkmate?
Answers.UserDisplayName: hkBst
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Answers.Text: That was my question too.
Answers.UserDisplayName: Jake B
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Answers.CreationDate: 2016-03-20T19:19:26.857Z
Answers.Text: There are positions where you must check a player already in check.  Logic would dictate that you are able to checkmate a player already in checkmate.
Answers.UserDisplayName: Mike Jones
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Answers.CreationDate: 2016-04-22T07:29:00.477Z
Answers.Text: Even if you make an unrelated move, the checkmate still stands, so in some sense you are also giving checkmate just by not removing it.
Answers.UserDisplayName: hkBst
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Answers.UserDisplayName: Mike Jones
Body: <p>My friends and I have been playing knightmare chess in a four player variant. We were playing with the stipulation that when a player is checkmated, after his turn he removes his king from the board and his pieces change allegiance to the last player to cause checkmate.</p>
  <p>I had caused checkmate on a player. An opponent between my turn and the checkmated player moved his bishop to cause check on the same king I had checkmated. He staked a claim to this player's pieces. I refuted his claim on the basis his bishop did not cause checkmate, only check while my pieces actually did cause checkmate. Who would get the pieces?</p>
  <p>By definition, check is a move where a peice is threatening on his opponents next turn. According to this, I argued that I caused checkmate and was entitled to his peices.</p>
  
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-03-12T00:22:16.650Z
Comments.Text: Does this four player variant of knightmare chess that you are playing already have well-established rules, or is it something you and your friends improvised yourselves? If it's the latter, I'd guess only you could answer the question. If it's the former, could you give a link to what the written rules have to say?
Comments.UserDisplayName: ETD
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Comments.CreationDate: 2016-03-12T00:25:37.157Z
Comments.Text: I'm not really sure. Knightmare chess is played with 2 players. But knightmare chess aside, if this were normal 4 player chess, what would the outcome be?  I feel as though the same situation could occur.
Comments.UserDisplayName: Jake B
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-03-12T00:31:18.283Z
Comments.Text: I'd have basically the same question again: what constitutes "normal" four-player chess? I imagine there are a number of variants with different rule sets, [as indicated on Wikipedia for instance](https://en.wikipedia.org/wiki/Four-player_chess). My impression is that there is no single, standard set of rules to consult, and that you should then choose to fix whatever rules for this situation you can all agree on.
Comments.UserDisplayName: ETD
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-03-12T00:37:29.520Z
Comments.Text: Yes I've consulted this wiki article before. I guess it comes down to a small blurb under "Singles." What does "the player that checkmated" mean? That would be the first player to cause checkmate? Because he had not caused checkmate, only check. I know this is a pretty obscure question.
Comments.UserDisplayName: Jake B
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-03-19T11:33:38.190Z
Comments.Text: Do you have a rule for what happens when a king is checkmated by the combined pieces of two other players, but not by those of any single player?
Comments.UserDisplayName: hkBst
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-03-19T14:11:15.777Z
Comments.Text: It would be the last player to make a move that causes the check mate. I've been looking on the official rules but there isn't a lot of information on this.
Comments.UserDisplayName: Jake B
Tags: <checkmate><chess-variants>
Title: Four way chess -checkmate
ThreadText: # Four way chess -checkmate
  
  (Asked by: username_0 on 2016-03-11)
  
  My friends and I have been playing knightmare chess in a four player variant. We were playing with the stipulation that when a player is checkmated, after his turn he removes his king from the board and his pieces change allegiance to the last player to cause checkmate.
  
  I had caused checkmate on a player. An opponent between my turn and the checkmated player moved his bishop to cause check on the same king I had checkmated. He staked a claim to this player's pieces. I refuted his claim on the basis his bishop did not cause checkmate, only check while my pieces actually did cause checkmate. Who would get the pieces?
  
  By definition, check is a move where a peice is threatening on his opponents next turn. According to this, I argued that I caused checkmate and was entitled to his peices.
  
  **Tags:** checkmate, chess-variants
  
  > **Comments:**
  > **username_1**: Does this four player variant of knightmare chess that you are playing already have well-established rules, or is it something you and your friends improvised yourselves? If it's the latter, I'd guess only you could answer the question. If it's the former, could you give a link to what the written rules have to say?  
  > **username_0**: I'm not really sure. Knightmare chess is played with 2 players. But knightmare chess aside, if this were normal 4 player chess, what would the outcome be?  I feel as though the same situation could occur.  
  > **username_1**: I'd have basically the same question again: what constitutes "normal" four-player chess? I imagine there are a number of variants with different rule sets, [as indicated on Wikipedia for instance](https://en.wikipedia.org/wiki/Four-player_chess). My impression is that there is no single, standard set of rules to consult, and that you should then choose to fix whatever rules for this situation you can all agree on.  
  > **username_0**: Yes I've consulted this wiki article before. I guess it comes down to a small blurb under "Singles." What does "the player that checkmated" mean? That would be the first player to cause checkmate? Because he had not caused checkmate, only check. I know this is a pretty obscure question.  
  > **username_2**: Do you have a rule for what happens when a king is checkmated by the combined pieces of two other players, but not by those of any single player?  
  > **username_0**: It would be the last player to make a move that causes the check mate. I've been looking on the official rules but there isn't a lot of information on this.  
  
  ---
  
  ## 3 answers
  
  ### Answer by username_2 on 2016-04-22. Score: 3
  
  I will need to make an educated guess as to what rules come closest to what you have described, and I think it is that:
  
  1. You are checkmated whenever your king is attacked and if it were your turn next, you would not have any moves that would result in your king not being attacked.
  2. An opponent causes checkmate on you, if at the start of their turn you are not checkmated, but at the end of their turn you are.
  3. If at the start of your own turn you are checkmated, then your king is removed from the game and the opponent who last caused checkmate on you will gain control of your pieces.
  
  Under these rules, merely piling on a check while it was already checkmate would not cause checkmate.
  
  The rest of this post is background and further analysis of checkmate, that informed my answer and may help to understand it:
  
  According to [Wikipedia](https://en.wikipedia.org/wiki/Checkmate#History), it was the Persians who abandoned king capture and introduced warning of check in order to not accidentally end the game. So let's go back in time before the introduction of checkmate and reanalyse.
  
  In two-player chess checkmate is a situation where at the end of your turn all parties are certain that you will (be able to) capture the king on your very next turn (and in two-player chess there exist no reasons not to actually capture the king). Because of this certainty of how the next two turns will play out (a doomed evasive move and a certain king capture), there is no point in actually playing those moves, so both can be (and have been in FIDE chess) eliminated from the game with no detriment to the game. (Very much the same thing happens when a player forfeits the game upon realizing that the game is lost and there is no point playing any further.)
  
  But if there are three or more players a third party can intervene on behalf of a threatened king and (unless the game ends when the first king is captured) there may be more pressing matters than capturing an opponent's king left in check (like defending your own king). These differences make it impossible to say with certainty when king capture is actually going to take place before it actually happens, thereby invalidating the reasoning which allowed us to eliminate king capture from the (end of the) game.
  
  It is of course possible to try and use a definition of checkmate that is equivalent to the full checkmate described above in two-player chess also in chess with three or more players. Indeed checkmate is usually defined in an operational way that is equivalent to full checkmate in two-player chess: your king is attacked and you have no move that will result in your king not being attacked.
  
  When you try to use that definition in multi-player chess you will need to think about on which player's turns one can be checkmated. If your king is attacked and you have no move that will result in your king not being attacked, because it is not even your turn, are you checkmated? Apparently that was something you rejected, but then it becomes hard to say who checkmated you. It is possible for example that the player before you causes your king to be attacked by moving one of their pieces out of the way of a third player's piece, and even causes you to not have any moves that result in your king not being attacked, even though they are not even checking you with any of their own pieces. Have they checkmated you? I think most people are inclined to say yes. But if you want to know if they caused checkmate or not, then you need to be able to say whether or not you were already checkmated before their turn or not. Thus you need a way to determine checkmate even when it is not your turn.
  
  One extreme is to say that every check from a player not immediately before you is checkmate (because as it is not even your turn you have no moves to remove that attack). Another way is to pretend that it were your turn and see if you could do something. I have recently even read rules for multi-player chess where it actually becomes your turn if you are checked, and the player whose turn it would have been had you not been checked continues after you (and you may or may not forfeit you actual next turn).
  
  
  ---
  
  ### Answer by username_3 on 2021-02-04. Score: 1
  
  I nearly agree with @username_2, who says that one should determines the beneficiary at the beginning of the mated player's turn, by asking who would be able to actually capture the king (in the pre-Persian format). This would certainly give a unique result.
  
  The removal of the king may also cause discovered checkmates to happen to as many a 4 other players in the game, if the board were large enough. And when they are eliminated for each of them that could result in another 3 discovered checkmates happening. Thus a cascade of checkmates might happen and the original mater might never have to move again.
  
  Alternatively, the elimination of the king might create a discovered mate of the original mater himself! Does this mean that the original mate was illegal? I don't think so, in that the removal of the king in this case behaves like the mated player's move.
  
  I also can't resist pointing out that the questioner says that under their rules, the mated king is removed **after** that player's turn. This suggests that the player gets to make some "last dying move", illegal in the sense that it doesn't avoid check, because it can't.
  
  This would allow a mated player in some circumstances to select which of the opponents would inherit all the pieces. In a multi-player game like Diplomacy, selecting which of one's attackers will be the beneficiary is a very real threat, which can discourage attack and keep small fry alive. This is perhaps the most apposite possible example of the chess saying that the threat is more powerful than its execution, in that this can only be triggered if the player has definitively lost.
  
  Can one capture pieces one has inherited, or are they now part of one's own force? If the latter, then it's possible that one might be stalemated as a result of inheriting pieces that one can no longer capture.
  
  Finally, how *do* stalemates work in this game? Are the pieces frozen? Can they re-awake later potentially? Does the stalemated player need to hang around in case they are brought back to life?
  
  So many questions about this brilliant mechanism...
  
  
  ---
  
  ### Answer by username_4 on 2016-03-12. Score: 1
  
  Checkmate is when the King is in check and no move would be able to remove him from check. If the Bishop check is a move that delivered checkmate, even if he was checkmated before, is the last checkmate and the pieces would be his.
  
  > **Comments:**
  > **username_2**: How can you deliver checkmate when it is already checkmate?  
  > **username_0**: That was my question too.  
  > **username_4**: There are positions where you must check a player already in check.  Logic would dictate that you are able to checkmate a player already in checkmate.  
  > **username_2**: Even if you make an unrelated move, the checkmate still stands, so in some sense you are also giving checkmate just by not removing it.  
  
  ---
  
  