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Chess has a rule that forbids castling when in check. What is the practical value of this rule? How would chess without this rule compare to chess with this rule?

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    I have removed the "opinion based" part of the question and suggest that it NOT be closed. What's left can be answered by reference to the chess literature. – Tom Au Jan 31 '16 at 21:35
  • @TomAu I don't think it should be closed but to me this is pretty much the same question. It's not asking "why/when was this rule added". To answer the question as asked, it's still effectively going to be a comparison of chess now to what modern chess might look like without the rule. – Samthere Feb 1 '16 at 9:57
  • However I feel that this question is suitable as a subjective question. – Samthere Feb 1 '16 at 9:59
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One important reason to check a king is to prevent him from castling later by forcing him to move. The reason is that a king that has already moved (and remains in the center) cannot later "castle" his way into relative safety on the side. Nor can a king castle while in check, so this prevents him from castling for at least that move.

There are two other ways to defend a king in check without moving him. One is to capture the checking piece, and the other is to "interpose" another piece between the checking piece and the king. If the king can then be prevented from moving, he can castle later. But these remedies expose the defending side to other risks, which are probably included in the reasons to check the king.

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    Answer doesn't address the question, just states the circumstances in which Castling can be prevented. – Drunk Cynic Feb 1 '16 at 0:12
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    I think this does somewhat explain what was intended even if it does not fully explain it. – Joe W Feb 1 '16 at 0:17
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    So if I am reading you correctly, you're saying that the prohibition on the king castling out of check, makes it possible to attack the a king that is ready to castle, possibly preventing the castling move. If this prohibition were to be dropped, it would be harder to prevent castling and thus the prospects for a player going on the attack would be reduced. So what I am really wondering about is whether there is some evidence of this in chess openings or tournament play. – hkBst Feb 2 '16 at 8:17
  • @hkBst: You're reading my answer correctly. From what I understand, the "law" was established a long time (over 100 years) ago. So it may or may not be supported by recent results or tournament play. – Tom Au Feb 2 '16 at 13:20
  • By chance I ran into this game of Rybka (Computer) vs Deep Sjeng (Computer) 17th World Computer Chess Championship (2009) · Slav Defense: Chameleon Variation (D15) · 1-0 where 15 O-O is illegal due to being in check. – hkBst Feb 16 '16 at 8:29
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Per the wikipedia page for castling, it appears that castling arose in response to the growth in the power of the bishop and queen move sets. The line that is most interesting is a reference to the Gottingen Manuscript:

In the Göttingen manuscript (c. 1500) and a game published by Luis Ramírez de Lucena in 1498, castling consisted of two moves: first the rook and then the king.

If this is correct, than 'casting' while in check would entail moving the rook first, and then 'leaping' the rook with the king. If the king is in check when you attempt to castle, you would be moving the rook with the king still in check.

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