Answers.Body: <p>For this kind of question, if you have doubts, just play chess with the goal of capturing the enemy king. The first player to capture the enemy king wins. This isn't the official rule, but it's effectively the same in almost all situations I'm aware of (it doesn't work for stalemate).</p>
  <p>So after 1...Rf8+, White wins with 2. Rxd2. It doesn't matter that Black also threatens to capture White's king with 2...Rxa8, since White has already won.</p>
  
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Answers.Text: Its better to clarify that a rule of chess force to get out of check, block the check or capture the piece giving check BEFORE anything else.
Answers.UserDisplayName: djnavas
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Answers.Text: It may be worth noting that the rules differ from a game that doesn't recognize check or checkmate, but players win by capturing the enemy king, in one important way: a player cannot be compelled to move his king into danger; if a player's king is not in immediate danger, but all moves would endanger it, the game is considered stalemated and scored as a draw rather than for a win by the player who could otherwise capture the king.  This exception may seem subtle, but it greatly affects the range of endgames where checkmate can be forced.
Answers.UserDisplayName: supercat
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Answers.Text: @supercat That's already covered by the answer: "(It doesn't work for stalemate)"
Answers.UserDisplayName: amalloy
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Answers.Text: I've played **many** games where the goal really was capturing the king--when we were playing 5 minute games things--including threats to the king--sometimes get overlooked.  Check had no special status, the game ended with a king capture.  I never saw a game where the winner lost on time in a checkmate position but I did see a discovered check persist for IIRC 4 moves, neither side aware of it.
Answers.UserDisplayName: Loren Pechtel
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Answers.Text: The key part of this question is the terms “check” and “checkmate”, implying that standard tournament rules apply. Also, back when I was at a chess club, there was some confusion in rapid games about capturing the opponent’s king after the opponent failed to stop a check. Was it a claim of an illegal move by the opponent (winning the game) or an illegal move itself (losing the game, if the opponent immediately made such a claim)? They eventually clarified that it was the latter. For these reasons, the advice in this answer seems very questionable in this context.
Answers.UserDisplayName: Brian Drake
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Answers.Text: @BrianDrake [Well yes](https://chess.stackexchange.com/questions/441/is-taking-the-opponents-king-an-illegal-move) but the question only asks if the move in question is legal. It isn't, since White captures the king.
Answers.UserDisplayName: Allure
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Answers.Text: I don't think it's that helpful to offer a simplified heuristic like this in response to a question about the rules. For instance, this heuristic would give you the wrong answer in the case of castling across check, or castling out of check.
Answers.UserDisplayName: Steve Bennett
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Answers.Text: To drive the point home, consider your king as the one commanding the army.  When he's removed from the board, the remaining pieces have no leader and nobody to tell them to move.
Answers.UserDisplayName: Michael
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Answers.Text: This would also open the option of playing to capture the king, and giving the loser one move after that happens to force a tie (momentum of battle and all that.) Might make for some interesting strategies.
Answers.UserDisplayName: Perkins
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Answers.Text: @djnavas: I find that describing the rule this way is needlessly contrived. The more straightforward interpretation is that _not_ getting out of/blocking/capturing the check logically throws the game as the opponent can therefore "commit" the check into a capture. Therefore, anything that does not remove the check can be interpreted as willful resignation. Making it illegal to do so simply forces the player to expressly state that they resign, as opposed to making a move and then _inferring_ that they may have intentionally resigned the game.
Answers.UserDisplayName: Flater
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Answers.Text: @Flater You say “needlessly contrived”; I say “accurate” (albeit poorly worded: “get out of check” is a generic phrase; it should be more like “move the king out of check”). When a player fails to stop a check, this is usually not deliberate, so inferring that the player intended to resign is absurd. If failing to stop a check were legal, then it would be more likely to be deliberate, but it would still not be resignation, just as continuing play in any other “lost” position is not resignation.
Answers.UserDisplayName: Brian Drake
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Answers.Text: @Michael I am not sure whether that analogy is helpful either. See [Qwert Yuiop’s answer](https://chess.stackexchange.com/a/39534/25131) and my comment there.
Answers.UserDisplayName: Brian Drake
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Answers.CreationDate: 2022-03-26T06:53:50.600Z
Answers.Text: -1 To drive the *real* point home, this answer’s reasoning is wrong, as [pointed out by Steve Bennett](https://chess.stackexchange.com/questions/39499/if-player-a-is-in-check-but-has-a-move-which-will-checkmate-player-b-without-exi/39510#comment63595_39501) and others. As I post this comment, this answer has a score of 36, just behind the [top answer](https://chess.stackexchange.com/a/39502/25131) (score: 38). So, clearly, the community sees some value here, but I just can’t see it.
Answers.UserDisplayName: Brian Drake
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Answers.Text: @BrianDrake The absurdity of wilfully not getting out of check is _precisely_ why it can be equated to a resignation. It is a conscious decision to make it so the game _will_ be won by the opponent with no more recourse.
Answers.UserDisplayName: Flater
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Answers.CreationDate: 2022-04-10T21:56:28.763Z
Answers.Text: @BrianDrake Castling is the exception here: the heuristic is excellent in general. The castling-specific rules are oddities somewhat, nor does the fact that you can't castle out of check (or through check) help understand why one must move out of check when they could deliver mate instead.
Answers.UserDisplayName: Mobeus Zoom
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Answers.CreationDate: 2024-01-14T21:48:33.763Z
Answers.Text: And the rules about stalemate were added so the "clear winner" still needs to pay attention. The fact it got accepted as legit defense tactics (e.g. K vs Kp is draw even if both play perfectly) is a byproduct.
Answers.UserDisplayName: Ferazhu
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Answers.LastActivityDate: 2022-03-22T07:07:13.183Z
Answers.UserDisplayName: Allure
Answers.Body: <p>According the <a href="https://handbook.fide.com/chapter/E012018" rel="nofollow noreferrer">FIDE LAWS of CHESS</a>:</p>
  <blockquote>
  <p>3.9.2 <strong>No piece can be moved that will</strong> either expose the king of the same colour to check or <strong>leave that king in check.</strong></p>
  </blockquote>
  <p>So as @djnavas points out, if you are in check you have to do at least one of three things (without placing the king in another check) to <a href="https://en.wikipedia.org/wiki/Check_(chess)" rel="nofollow noreferrer">get out of check</a> and continue the game:</p>
  <ul>
  <li>move the king out of check,</li>
  <li>capture the piece checking you, or</li>
  <li>block the check (by putting one of your pieces in between the king and the piece checking you).</li>
  </ul>
  
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Answers.Text: And of course, (4) resign.
Answers.UserDisplayName: Chuu
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Answers.Text: @Chuu 5) Mutually agree the game is a draw.
Answers.UserDisplayName: Patrick M
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Answers.Text: The "you have to do one of three things" is a bit incomplete, because doing one of those three things may not be enough. And in some cases, doing one of them (say, capturing the piece checking you) may open up a different check, which is not legal.
Answers.UserDisplayName: Steve Bennett
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Answers.Text: @SteveBennett We can say that doing at least one of these things is *necessary* to continue the game, but may not be *sufficient*. That is enough to answer the question: the move proposed in the question is not legal. Now, why might it not be sufficient? Perhaps because a different check was opened up. Perhaps because this is the second time the player has used two hands (which is now treated as an illegal move under FIDE rules). Perhaps because they ran out of time. Perhaps some other reason.
Answers.UserDisplayName: Brian Drake
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Answers.Text: @PatrickM LOL! although I suppose it's possible that one might agree to a draw when they are currently checking their opponent in some circumstances.
Answers.UserDisplayName: Michael
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Answers.Text: @Michael Presumably, those circumstances include perpetual check, which seems like a likely outcome of this position.
Answers.UserDisplayName: Brian Drake
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Answers.CreationDate: 2022-03-28T10:33:58.763Z
Answers.Text: @Michael This is actually such a circumstance. White will keep checking and force black to either take the rook (causing stalemate) or repeat moves.
Answers.UserDisplayName: Comic Sans Seraphim
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Answers.UserDisplayName: Snostorp
Answers.Body: <p>No, this is not legal. The fact that the black king is still in check from the white rook overrides every other consideration. It doesn't matter that (were it not for the white rook) Black's move would be checkmate, it fails to get Black out of check, so it's not legal.</p>
  
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Answers.CreationDate: 2022-03-26T06:29:52.577Z
Answers.Text: +1 This answer looks redundant to other answers, but this was not the case when it was posted. In fact, out of the answers currently visible, this was the first one to be posted.
Answers.UserDisplayName: Brian Drake
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Answers.UserDisplayName: Rosie F
Answers.Body: <p><strong>No, this move (...Rf8#) is not legal.</strong> According to the <a href="https://handbook.fide.com/chapter/E012018" rel="noreferrer">FIDE Laws of Chess effective 1 January 2018</a>:</p>
  <blockquote>
  <p>3.10.2 A move is illegal when it fails to meet the relevant requirements of Articles 3.1 – 3.9</p>
  </blockquote>
  <p>(This improves on <a href="https://handbook.fide.com/chapter/E01pre2014" rel="noreferrer">pre-July 2014 rules</a> by specifically defining “illegal” as well as “legal”.)</p>
  <p>In this case, the relevant requirement is in Article 3.9.2:</p>
  <blockquote>
  <p>No piece can be moved that will … leave [the king of the same colour] in check.</p>
  </blockquote>
  <p>The consequence of this is that <strong>the checkmate does not count</strong>:</p>
  <blockquote>
  <p>5.1.1 The game is won by the player who has checkmated his opponent’s king. This immediately ends the game, provided that the move producing the checkmate position was in accordance with Article 3 …</p>
  </blockquote>
  <p>Instead, the game shall continue with a legal move (after reverting the illegal move):</p>
  <blockquote>
  <p>7.5.1 An illegal move is completed once the player has pressed his clock. If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. … Article[] 4.3 … appl[ies] to the move replacing the illegal move. The game shall then continue from this reinstated position.</p>
  </blockquote>
  <p>Article 4.3 is the so-called “touch-move” rule and, in this case, would require Black to play ...Rf2 (probably losing the rook on the next move).</p>
  <p>Actually, the key point here is also covered in the introductory Article 1:</p>
  <blockquote>
  <p>Leaving one’s own king under attack … is not allowed .</p>
  </blockquote>
  <p>As an aside, ...Rb8# would also be checkmate, but would also be illegal. This rook has no legal moves, so “touch-move” would not be relevant here and Black would be able to make any legal move after the illegal move was reverted.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2022-03-23T12:09:17.663Z
Answers.Text: This is a good answer that actually refers to the rules.
Answers.UserDisplayName: Steve Bennett
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Answers.LastEditDate: 2022-03-23T13:13:32.410Z
Answers.UserDisplayName: Brian Drake
Answers.Body: <p>In the case of the 'primitive' form of chess, where leaving your king in check is not an 'illegal' move, your king would then be taken by an intelligent player, causing you to lose.</p>
  <p>The fact that be you could take his king the next turn would not change this.</p>
  <p>However, in 'modern' chess, having your king captured is illegal(The game ends at checkmate, where you have no way to get out of check).
  Since your king could be taken the next turn, you remain in check, and your only available move is one which gets you out of check, i.e., makes sure your king cannot be taken the next turn.</p>
  <p>You might think that putting the other player in check(or checkmate) would make it so he could not take your king, but the same rule which binds him also binds you, so <em>you</em> also cannot place his king in check while your king is in check, unless you place his king in check(or checkmate) while getting your king out of check at the same time, which is not the case here.</p>
  
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Answers.UserDisplayName: LWS SWL
Answers.Body: <p>I mean, I'm no avid chess player, take what I say with a grain of salt, as I'm only trying to apply logic here.
  The point of check is that in the next turn, if you don't solve that situation, you lose.
  So, if you ignore check, and produce checkmate while you're still in check, it only means that YOUR next turn the enemy loses 100%.
  But you lose before that, as it's the enemy's turn and they take your king before proceeding with your play.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2022-03-26T06:22:21.947Z
Answers.Text: This is basically the same as [Allure’s answer](https://chess.stackexchange.com/a/39501/25131).
Answers.UserDisplayName: Brian Drake
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Answers.LastEditDate: 2022-03-24T13:17:25.380Z
Answers.UserDisplayName: user32298
Answers.Body: <p>No, that's not a legal move. When you are in check, you have to do anything that will stop you being in check, so any of:</p>
  <ul>
  <li>Move king to a field not attackable by opponent pieces.</li>
  <li>Move one of your non-king pieces in between to block the attack path. Not possible against the horse because it jumps.</li>
  <li>Capture the opponent's piece that is threatening your king.</li>
  </ul>
  <p>The real world analogy to your situation is that you are the leader and you are in the enemy sniper's scope and you know it. Your reaction is to move your men into position to kill the enemy leader. While your men are moving into position to surround the enemy leader, their sniper kills you. So even if hypothetically your men manage to later kill their leader, you still died first. You effectively committed suicide for a chance to take the enemy leader down with you.</p>
  
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Answers.CreationDate: 2022-03-26T06:14:35.543Z
Answers.Text: The first part of this answer mainly restates other answers, but in a poorly worded way. I do not know what “field” is supposed to mean. The piece on c6 is properly known as a “knight”, not a “horse” ([FIDE Laws of Chess effective 1 January 2018](https://handbook.fide.com/chapter/E012018), Article 2.2). I am not sure that the analogy is helpful: checks are common, especially among low-level players, and not necessarily indicative of the state of the game, while it is rare for leaders to be in mortal danger, and even more rare for the leaders on both sides to be in mortal danger simultaneously!
Answers.UserDisplayName: Brian Drake
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Answers.LastEditDate: 2022-03-25T11:06:03.780Z
Answers.UserDisplayName: Qwert Yuiop
Answers.Body: <p>No. If you are in check, you have to get out of check, either by moving king or blocking check with other piece. Anything else is illegal.</p>
  
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Answers.CreationDate: 2022-03-23T10:44:57.320Z
Answers.Text: I'm sorry but this doesn't add anything new that hasn't already been answered by someone else. And you also forgot that you can always capture the piece checking you.
Answers.UserDisplayName: Snostorp
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Answers.UserDisplayName: Amit G
Body: <p>Consider a player who is in check. Suppose they can make a move that would checkmate the opponent, but would not stop themselves from being in check. Would that be a legal move?</p>
  <p>For example, in the position below, if Black plays ...<code>Rf8</code>, it will put the White king in checkmate, but the Black king will still be in check. Is this move legal?</p>
  <pre><code>[FEN &quot;K7/8/2n5/1r6/8/8/3k3R/5r2 b - - 1 0&quot;]
  [StartFlipped &quot;0&quot;]
  </code></pre>
  <p>Note that this position is displayed from White’s point of view, even though it is Black’s move.</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-03-22T13:51:52.287Z
Comments.Text: This is not putting white in checkmate.
Comments.UserDisplayName: TylerH
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-03-23T09:38:47.837Z
Comments.Text: @TylerH It certainly looks it t me - why isn't it? What valid move does the white king have?
Comments.UserDisplayName: Steve Ives
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-03-23T15:22:28.527Z
Comments.Text: @SteveIves Checkmate requires no escape from check. White can escape from check by taking Black's king and winning the game. QED, not checkmate. (To be clear, my comment above is intended as a bit of a frame challenge by highlighting that the language used is problematic and that's why OP has this confusion in the first place).
Comments.UserDisplayName: TylerH
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-03-24T11:42:55.157Z
Comments.Text: I'm with @TylerH  here.  The situation you are trying to describe can't happen, because by definition the only way to avoid having your king captured on the next move is to escape check.
Comments.UserDisplayName: Carl Witthoft
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-04-04T13:42:34.967Z
Comments.Text: @TylerH Got it - so yes, " Black plays ...Rf8" would be checkmate, were it a valid move.
Comments.UserDisplayName: Steve Ives
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-04-05T16:41:34.357Z
Comments.Text: @SteveIves "I could win if I could perform this illegal move" is not a useful/valid argument in determining the outcome of a Chess game.
Comments.UserDisplayName: TylerH
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-04-07T09:26:30.287Z
Comments.Text: @TylerH My question was actually "Why is Rf8 not checkmate?" in response to the statement that it wasn't (and the answer is that it would be) rather than if that was a legal move.
Comments.UserDisplayName: Steve Ives
Tags: <rules><checkmate><check><winning>
Title: Can a player in check win, by checkmating the opponent, while not getting themselves out of check?
ThreadText: # Can a player in check win, by checkmating the opponent, while not getting themselves out of check?
  
  (Asked by: username_0 on 2022-03-22)
  
  Consider a player who is in check. Suppose they can make a move that would checkmate the opponent, but would not stop themselves from being in check. Would that be a legal move?
  
  For example, in the position below, if Black plays ...`Rf8`, it will put the White king in checkmate, but the Black king will still be in check. Is this move legal?
  
  ```
  [FEN "K7/8/2n5/1r6/8/8/3k3R/5r2 b - - 1 0"]
  [StartFlipped "0"]
  
  ```
  
  Note that this position is displayed from White’s point of view, even though it is Black’s move.
  
  **Tags:** rules, checkmate, check, winning
  
  > **Comments:**
  > **username_1**: This is not putting white in checkmate.  
  > **username_2**: @username_1 It certainly looks it t me - why isn't it? What valid move does the white king have?  
  > **username_1**: @username_2 Checkmate requires no escape from check. White can escape from check by taking Black's king and winning the game. QED, not checkmate. (To be clear, my comment above is intended as a bit of a frame challenge by highlighting that the language used is problematic and that's why OP has this confusion in the first place).  
  > **username_3**: I'm with @username_1  here.  The situation you are trying to describe can't happen, because by definition the only way to avoid having your king captured on the next move is to escape check.  
  > **username_2**: @username_1 Got it - so yes, " Black plays ...Rf8" would be checkmate, were it a valid move.  
  > **username_1**: @username_2 "I could win if I could perform this illegal move" is not a useful/valid argument in determining the outcome of a Chess game.  
  > **username_2**: @username_1 My question was actually "Why is Rf8 not checkmate?" in response to the statement that it wasn't (and the answer is that it would be) rather than if that was a legal move.  
  
  ---
  
  ## 8 answers
  
  ### Answer by username_4 on 2022-03-22. Score: 39
  
  For this kind of question, if you have doubts, just play chess with the goal of capturing the enemy king. The first player to capture the enemy king wins. This isn't the official rule, but it's effectively the same in almost all situations I'm aware of (it doesn't work for stalemate).
  
  So after 1...Rf8+, White wins with 2. Rxd2. It doesn't matter that Black also threatens to capture White's king with 2...Rxa8, since White has already won.
  
  > **Comments:**
  > **username_5**: Its better to clarify that a rule of chess force to get out of check, block the check or capture the piece giving check BEFORE anything else.  
  > **username_6**: It may be worth noting that the rules differ from a game that doesn't recognize check or checkmate, but players win by capturing the enemy king, in one important way: a player cannot be compelled to move his king into danger; if a player's king is not in immediate danger, but all moves would endanger it, the game is considered stalemated and scored as a draw rather than for a win by the player who could otherwise capture the king.  This exception may seem subtle, but it greatly affects the range of endgames where checkmate can be forced.  
  > **username_7**: @username_6 That's already covered by the answer: "(It doesn't work for stalemate)"  
  > **username_8**: I've played **many** games where the goal really was capturing the king--when we were playing 5 minute games things--including threats to the king--sometimes get overlooked.  Check had no special status, the game ended with a king capture.  I never saw a game where the winner lost on time in a checkmate position but I did see a discovered check persist for IIRC 4 moves, neither side aware of it.  
  > **username_9**: The key part of this question is the terms “check” and “checkmate”, implying that standard tournament rules apply. Also, back when I was at a chess club, there was some confusion in rapid games about capturing the opponent’s king after the opponent failed to stop a check. Was it a claim of an illegal move by the opponent (winning the game) or an illegal move itself (losing the game, if the opponent immediately made such a claim)? They eventually clarified that it was the latter. For these reasons, the advice in this answer seems very questionable in this context.  
  > **username_4**: @username_9 [Well yes](https://chess.stackexchange.com/questions/441/is-taking-the-opponents-king-an-illegal-move) but the question only asks if the move in question is legal. It isn't, since White captures the king.  
  > **username_10**: I don't think it's that helpful to offer a simplified heuristic like this in response to a question about the rules. For instance, this heuristic would give you the wrong answer in the case of castling across check, or castling out of check.  
  > **username_11**: To drive the point home, consider your king as the one commanding the army.  When he's removed from the board, the remaining pieces have no leader and nobody to tell them to move.  
  > **username_12**: This would also open the option of playing to capture the king, and giving the loser one move after that happens to force a tie (momentum of battle and all that.) Might make for some interesting strategies.  
  > **username_13**: @username_5: I find that describing the rule this way is needlessly contrived. The more straightforward interpretation is that _not_ getting out of/blocking/capturing the check logically throws the game as the opponent can therefore "commit" the check into a capture. Therefore, anything that does not remove the check can be interpreted as willful resignation. Making it illegal to do so simply forces the player to expressly state that they resign, as opposed to making a move and then _inferring_ that they may have intentionally resigned the game.  
  > **username_9**: @username_13 You say “needlessly contrived”; I say “accurate” (albeit poorly worded: “get out of check” is a generic phrase; it should be more like “move the king out of check”). When a player fails to stop a check, this is usually not deliberate, so inferring that the player intended to resign is absurd. If failing to stop a check were legal, then it would be more likely to be deliberate, but it would still not be resignation, just as continuing play in any other “lost” position is not resignation.  
  > **username_9**: @username_11 I am not sure whether that analogy is helpful either. See [username_23 username_23’s answer](https://chess.stackexchange.com/a/39534/25131) and my comment there.  
  > **username_9**: -1 To drive the *real* point home, this answer’s reasoning is wrong, as [pointed out by username_2 username_10](https://chess.stackexchange.com/questions/39499/if-player-a-is-in-check-but-has-a-move-which-will-checkmate-player-b-without-exi/39510#comment63595_39501) and others. As I post this comment, this answer has a score of 36, just behind the [top answer](https://chess.stackexchange.com/a/39502/25131) (score: 38). So, clearly, the community sees some value here, but I just can’t see it.  
  > **username_13**: @username_9 The absurdity of wilfully not getting out of check is _precisely_ why it can be equated to a resignation. It is a conscious decision to make it so the game _will_ be won by the opponent with no more recourse.  
  > **username_14**: @username_9 Castling is the exception here: the heuristic is excellent in general. The castling-specific rules are oddities somewhat, nor does the fact that you can't castle out of check (or through check) help understand why one must move out of check when they could deliver mate instead.  
  > **username_15**: And the rules about stalemate were added so the "clear winner" still needs to pay attention. The fact it got accepted as legit defense tactics (e.g. K vs Kp is draw even if both play perfectly) is a byproduct.  
  
  ---
  
  ### Answer by username_16 on 2022-03-22. Score: 37
  
  According the [FIDE LAWS of CHESS](https://handbook.fide.com/chapter/E012018):
  
  > 3.9.2 **No piece can be moved that will** either expose the king of the same colour to check or **leave that king in check.**
  
  So as @username_5 points out, if you are in check you have to do at least one of three things (without placing the king in another check) to [get out of check](https://en.wikipedia.org/wiki/Check_(chess)) and continue the game:
  
  * move the king out of check,
  * capture the piece checking you, or
  * block the check (by putting one of your pieces in between the king and the piece checking you).
  
  > **Comments:**
  > **username_17**: And of course, (4) resign.  
  > **username_18**: @username_17 5) Mutually agree the game is a draw.  
  > **username_10**: The "you have to do one of three things" is a bit incomplete, because doing one of those three things may not be enough. And in some cases, doing one of them (say, capturing the piece checking you) may open up a different check, which is not legal.  
  > **username_9**: @username_10 We can say that doing at least one of these things is *necessary* to continue the game, but may not be *sufficient*. That is enough to answer the question: the move proposed in the question is not legal. Now, why might it not be sufficient? Perhaps because a different check was opened up. Perhaps because this is the second time the player has used two hands (which is now treated as an illegal move under FIDE rules). Perhaps because they ran out of time. Perhaps some other reason.  
  > **username_11**: @username_18 LOL! although I suppose it's possible that one might agree to a draw when they are currently checking their opponent in some circumstances.  
  > **username_9**: @username_11 Presumably, those circumstances include perpetual check, which seems like a likely outcome of this position.  
  > **username_19**: @username_11 This is actually such a circumstance. White will keep checking and force black to either take the rook (causing stalemate) or repeat moves.  
  
  ---
  
  ### Answer by username_20 on 2022-03-22. Score: 16
  
  No, this is not legal. The fact that the black king is still in check from the white rook overrides every other consideration. It doesn't matter that (were it not for the white rook) Black's move would be checkmate, it fails to get Black out of check, so it's not legal.
  
  > **Comments:**
  > **username_9**: +1 This answer looks redundant to other answers, but this was not the case when it was posted. In fact, out of the answers currently visible, this was the first one to be posted.  
  
  ---
  
  ### Answer by username_9 on 2022-03-23. Score: 6
  
  **No, this move (...Rf8#) is not legal.** According to the [FIDE Laws of Chess effective 1 January 2018](https://handbook.fide.com/chapter/E012018):
  
  > 3.10.2 A move is illegal when it fails to meet the relevant requirements of Articles 3.1 – 3.9
  
  (This improves on [pre-July 2014 rules](https://handbook.fide.com/chapter/E01pre2014) by specifically defining “illegal” as well as “legal”.)
  
  In this case, the relevant requirement is in Article 3.9.2:
  
  > No piece can be moved that will … leave [the king of the same colour] in check.
  
  The consequence of this is that **the checkmate does not count**:
  
  > 5.1.1 The game is won by the player who has checkmated his opponent’s king. This immediately ends the game, provided that the move producing the checkmate position was in accordance with Article 3 …
  
  Instead, the game shall continue with a legal move (after reverting the illegal move):
  
  > 7.5.1 An illegal move is completed once the player has pressed his clock. If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. … Article[] 4.3 … appl[ies] to the move replacing the illegal move. The game shall then continue from this reinstated position.
  
  Article 4.3 is the so-called “touch-move” rule and, in this case, would require Black to play ...Rf2 (probably losing the rook on the next move).
  
  Actually, the key point here is also covered in the introductory Article 1:
  
  > Leaving one’s own king under attack … is not allowed .
  
  As an aside, ...Rb8# would also be checkmate, but would also be illegal. This rook has no legal moves, so “touch-move” would not be relevant here and Black would be able to make any legal move after the illegal move was reverted.
  
  > **Comments:**
  > **username_10**: This is a good answer that actually refers to the rules.  
  
  ---
  
  ### Answer by username_21 on 2022-03-23. Score: 3
  
  In the case of the 'primitive' form of chess, where leaving your king in check is not an 'illegal' move, your king would then be taken by an intelligent player, causing you to lose.
  
  The fact that be you could take his king the next turn would not change this.
  
  However, in 'modern' chess, having your king captured is illegal(The game ends at checkmate, where you have no way to get out of check).
  Since your king could be taken the next turn, you remain in check, and your only available move is one which gets you out of check, i.e., makes sure your king cannot be taken the next turn.
  
  You might think that putting the other player in check(or checkmate) would make it so he could not take your king, but the same rule which binds him also binds you, so *you* also cannot place his king in check while your king is in check, unless you place his king in check(or checkmate) while getting your king out of check at the same time, which is not the case here.
  
  
  ---
  
  ### Answer by username_22 on 2022-03-24. Score: 0
  
  I mean, I'm no avid chess player, take what I say with a grain of salt, as I'm only trying to apply logic here.
  The point of check is that in the next turn, if you don't solve that situation, you lose.
  So, if you ignore check, and produce checkmate while you're still in check, it only means that YOUR next turn the enemy loses 100%.
  But you lose before that, as it's the enemy's turn and they take your king before proceeding with your play.
  
  > **Comments:**
  > **username_9**: This is basically the same as [username_4’s answer](https://chess.stackexchange.com/a/39501/25131).  
  
  ---
  
  ### Answer by username_23 on 2022-03-25. Score: -2
  
  No, that's not a legal move. When you are in check, you have to do anything that will stop you being in check, so any of:
  
  * Move king to a field not attackable by opponent pieces.
  * Move one of your non-king pieces in between to block the attack path. Not possible against the horse because it jumps.
  * Capture the opponent's piece that is threatening your king.
  
  The real world analogy to your situation is that you are the leader and you are in the enemy sniper's scope and you know it. Your reaction is to move your men into position to kill the enemy leader. While your men are moving into position to surround the enemy leader, their sniper kills you. So even if hypothetically your men manage to later kill their leader, you still died first. You effectively committed suicide for a chance to take the enemy leader down with you.
  
  > **Comments:**
  > **username_9**: The first part of this answer mainly restates other answers, but in a poorly worded way. I do not know what “field” is supposed to mean. The piece on c6 is properly known as a “knight”, not a “horse” ([FIDE Laws of Chess effective 1 January 2018](https://handbook.fide.com/chapter/E012018), Article 2.2). I am not sure that the analogy is helpful: checks are common, especially among low-level players, and not necessarily indicative of the state of the game, while it is rare for leaders to be in mortal danger, and even more rare for the leaders on both sides to be in mortal danger simultaneously!  
  
  ---
  
  ### Answer by username_24 on 2022-03-23. Score: -3
  
  No. If you are in check, you have to get out of check, either by moving king or blocking check with other piece. Anything else is illegal.
  
  > **Comments:**
  > **username_16**: I'm sorry but this doesn't add anything new that hasn't already been answered by someone else. And you also forgot that you can always capture the piece checking you.  
  
  ---
  
  