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I am confused why you cannot move into a check when it can force your opponent to capture your king and expose theirs. In such a case why is there no extra move to result in a draw?

In the "Racing Kings" variant, if white reaches the end of the board, black has an additional move to force a draw if they also reach the end of the board on that move. However, this does not seem to be the case in standard chess which seems to put black at a disadvantage. That is black must "defend" and white seems to have an advantage since it "attacks" from the very beginning of the game.

Here is an example of what I mean:

[fen "8/8/2k5/3R4/2r1PK2/8/8/8 w - - 0 1"]

If it is black to move, why can't they capture the rook (d5)? This would force white to recapture with the pawn and open up the king to a check from black's rook. Since black started second, it should have the chance to capture the king and force a draw.

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    When you ask "why is it illegal," are you asking what rule makes it illegal, or are you asking why the rules were designed so as to make it illegal? – Tanner Swett Dec 20 '19 at 17:15
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    There are loads of alternative chess variants with different board size or form, board setup, pieces, or rules. People settle for orthodox chess because it's interesting the way it is. – Quora Feans Dec 21 '19 at 0:12
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Imagine a variation of chess without the rules about check and checkmate, where a player wins simply when he captures his opponent's king. In this variation, Kxd5 loses the game to exd5.

Turns out, that's more or less how real chess works. The objective is to capture the opponent's king. If your king is under attack, you must deal with that threat. If there's no way to remove the threat, then you lose.

The explicit rule that you must end your move not in check (both by eliminating the check if you're in check, and by not moving into check) and the rule that ends the game with checkmate simply ensure that the king is never actually captured. Check can almost be seen as a safety measure for weaker players who might not notice that their king is under attack. Notice that announcing check is not actually in the rules, and grandmasters never do it. The rule is effectively unnecessary for experts. And checkmate is sort of like the game forcing a resignation when the result is inevitable, much like how experts will resign when they see that their opponent has forced mate.

If we think of the rules of check this way, then it's clear why Kxd5 is illegal: your opponent would take your king, and the fact that his own king becomes exposed is irrelevant because the game is over.

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Those are just the rules of the game. You could absolutely try to make the case that moving into check in such a situation should be legal, but playing by those rules wouldn't be chess anymore (it would be some variant).

You could also ask why stalemate is a draw and not a win, even though the latter result would make more sense in a real battle. These are simply the rules.

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I will answer from a different perspective: why Racing Kings (RK) has a rule to allow black a chance to draw, and why the same logic doesn't apply to chess.

What is Racing Kings (RK)?

Background for those unfamiliar with RK: Both sides start with all pieces (no pawns), arranged on the first 2 ranks of the chessboard, white on the right, black on the left. The win condition is to get the kings from their initial squares on h2/a2 to any square on the 8th rank. Checks are forbidden --- no move is allowed to leave either king in check.

The rules as stated above, favour white heavily. Having one tempo is critically important when a game can be as short as six moves. In order to negate some of this advantage, if white moves the king to the 8th rank and black can do the same immediately, the game is drawn.

How serious is white's advantage in RK?

Even with the added draw rule for black, RK still favours white. In practical play on lichess, eyeballing the opening explorer tells us white still wins more than black overall, even limiting the games to top players (2000+ site RK rating). The imbalance is around 48% to 41% with about 11% draws. This empirically shows white's advantage, and it would be even more lopsided without the draw rule. (And if there were long time control games, the draw percentage would be much, much higher. Such is internet blitz.)

Aside from game statistics, further evidence of the imbalance is the opening theory. 1. Kg3 lines have been very deeply analysed by the community and by Stockfish, and the outcome is that black can just about hold a draw with precise play (the theory can run 20 moves deep at times). The other best first moves 1. Bd4, 1. Nxc2 and 1. Kh3 are slightly less theoreticised, but deep engine analysis and practical play strongly suggests they are drawn too (again, with precise play).

So even with the draw rule in RK, black is still just holding on.

Should regular chess have this rule?

At the top level of regular chess, we have a far higher percentage of draws already. It is also clearly nowhere near as imbalanced as the "raw" form of RK without the draw rule. For balance reasons, the rule proposed here (allowing black to move into check in certain circumstances) should not be introduced to regular chess.

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It’s probable that White has some strategic advantage from going first. I never thought of your suggestion to give a losing player a “last-dying blow” might be a solution to this imbalance.

Now that I consider it, I think that it would not be desirable. Chess has too many draws anyway. That’s a much greater perceived issue than the white/black question. Your idea wouldn’t just turn White wins into draws but also Black wins.

Also, White’s advantage is strategic. If it translates into ability to checkmate with risk of last dying blow, then with a little more work it can probably translate into checkmate with no risk of last dying blow. So the issue and suggested solution are too separated

Hope this perspective helps.

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The question assumes that White has an inherent advantage and that therefore Black should have an extra chance to draw by "winning" on the same move number.

white seems to have an advantage since it "attacks" from the very beginning of the game

This is not true generically in games (https://en.wikipedia.org/wiki/First-player_and_second-player_win) - there are games where the second player can force a draw or even force a win. It is not proven which category chess falls into.

In any case, any advantage for white is not sufficiently large to make such a rule necessary. (Unlike racing kings where the advantage for white is considerable.)

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    It's generally accepted as statistically proven that White has small advantage in chess. Among good players, the statistics show the games split typically at around 55-45 for White (interestingly, in my coaching experience those numbers tip towards Black as the collective skill of the players drops). Which is not to say White has a forced win (though Hans Berliner and Weaver Adams have both insisted that was true, neither was able to successfully demonstrate it) just that White can afford to play less exactly than Black and still avoid losing. – Arlen Dec 31 '19 at 15:35
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Short answer is: In Classical Chess you are forbidden to put (or leave) your king in check (if you can't avoid it, you lose).

That's exactly the reason you can't put your king in check merrily (even in the event you think you can win by leaving your king in check "just for a moment") because you'd need an immediate second move to take it from danger (a second move that you don't have since in classical chess you can make only one move per turn).

Now, in the example you show, the above mentioned rule applies even if the offending piece (in this case a pawn) is pinned.

The reason is, the king is the only piece that never leaves the checkboard (can't be "captured"), so there's no need for the offending piece to take possession of the square the king is in (as it happens when capturing). In other words, immovility caused by the pin in this case is not a limiting issue and the above rule ("no king in check because") prevails.

(TBH long time ago I made myself this identical question ;-) )

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