user32279: 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.

user32291: 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.

user25131: No, this move (...Rf8#) is not legal. According to the FIDE Laws of Chess effective 1 January 2018:

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 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.

user10380: 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.

user32088: According the FIDE LAWS of CHESS:

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 @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 get out of check 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).


user15223: 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.

32279: <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>
 32291: <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>
 25131: <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>
 10380: <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>
 32088: <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>
 15223: <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>
