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Note: this is another rules-lawyering question.

I first ask for an official source I am calling into question the phrasing of the rules. Then I consider a few plausibly official sources, and point out a loophole (unsurprisingly not the first time castling has come under fire) - is my reading correct?

Preamble: Is there an official source for the rules on Chess960 castling?

Common knowledge at this point is that castling in Chess960 can be informally described as:

  • the king and rook end on the same squares as in regular chess,
  • the king and rook should not have moved before,
  • the squares the king and rook need to "slide through" must be empty,
  • the king should not be in, or pass through, or end in check.
  • (And the castling cannot be made with a promoted rook).

How about an official source?

Well, one would start by looking at the FIDE Laws of Chess as usual; the section Guidelines II is on Chess960 Rules. Disappointingly, only articles II.3.2.5.2 (specifying king and rook final squares) and II.3.2.7.4 (specifying passed-through squares must be vacant) seem relevant; no mention is made of the king being in or passing through check. So we assume that regular article 3.8.2 (specifically 3.8.2.2.1) applies to cover those.

Alternate sources would be Wikipedia (which cites for some reason dwheeler.com), and the organisers of the recent World Fischer Random Chess Championships, chess.com. Those read largely the same as the FIDE Laws.


The loophole

Consider the following position in Chess960. (wKb1, wRb1, bKe8, bRa1).

4k3/8/8/8/8/8/8/rR2K3 w Q - 0 1

Assume also that it's white's turn, and the white king and rook have never moved before. Can white castle left? Applying a little common sense, the answer is no (if we make the move, the white rook goes to d1 and the white king ends in check on c1).

However, according to every ruleset above (FIDE, Wikipedia, chess.com), all the criteria for castling have been met. In particular, in the diagram position, the black rook does not attack c1 nor d1, so the king is not moving through check. The relevant clause of the FIDE Laws is 3.8.2.2.1:

[Castling is prevented temporarily] if the square on which the king stands, or the square which it must cross, or the square which it is to occupy, is attacked by one or more of the opponent's pieces.

Which is not the case in the diagram.

One way out of this would be to invoke article 3.9.2 of the FIDE Laws:

No piece can be moved that will either expose the king of the same colour to check or leave that king in check.

Which by unfortunate phrasing, is ambiguous in the context of castling, which is "A move of the king and either rook of the same colour..." (Article 3.8.2), i.e. a move of two pieces. In the diagram position, moving the king alone, or the rook alone, will not expose the white king to check.


So I call attention to this loophole here. To summarise this question:

  1. Is there an official source for castling in Chess960?
  2. Is there an error in my reading of the rules that there is a loophole?
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Hi thanks for your questions Remellion. I have a better answer for Q2 than Q1, but bottom line there is no loophole in the FIDE Laws.

(1) I don't know of and can't locate any other official source of Chess960 rules other than FIDE rules.

(2) There is a second place in the FIDE rules where we are told how naughty it is to leave a king in check:

FIDE Law 1.4.1 Leaving one’s own king under attack, exposing one’s own king to attack and also ’capturing’ the opponent’s king is not allowed.

This would apply independently of the number of pieces involved in the castling. So there is no loophole.

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    I completely missed 1.4.1. That indeed plugs the hole. – Remellion Jan 6 at 13:32
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One way out of this would be to invoke article 3.9.2 of the FIDE Laws:

No piece can be moved that will either expose the king of the same colour to check or leave that king in check.

Yep, that seems entirely unambiguous to me. You can't leave your king in check after your move. (Your objection seems to be predicated on the idea that a move that touches two pieces can be said in some sense to have touched no pieces, and I think that's just wrong. You might with more grounding claim that article 3.9.2 permits you to end your turn in check if you got there by moving a pawn, since a pawn is "no piece"! :))

FIDE's Chess960 rules also state explicitly that when you castle, usually it's by moving two pieces one after the other:

II.3.2.1 double-move castling: by making a move with the king and a move with the rook, or

II.3.2.2 transposition castling: by transposing the position of the king and the rook, or

II.3.2.3 king-move-only castling: by making only a move with the king, or

II.3.2.4 rook-move-only castling: by making only a move with the rook.

So in your particular example, it's the "move with the rook" (II.3.2.1) that exposes its king to check (3.9.2) and is thus disallowed.

However, I admit that if you use that wording as gospel, then you have to point out that "transposing the position of the king and the rook" doesn't literally "move" either of the pieces; it just "transposes" them without using the word "move." So you could still ask your question about

4k3/8/8/8/8/8/8/r1RK4 w Q - 0 1

In that case, we'd just have to fall back on common sense: that you can't move your king into check, period. Notice that David Wheeler states that "the king may not be in check after castling" falls naturally out of the FIDE rules, without feeling the need to justify it any further:

[...] castling may only occur under the following conditions, which are extensions of the standard rules for castling:

  • Unmoved: The king and the castling rook must not have moved before in the game, including castling.
  • Unattacked: All of the squares between the king's initial and final squares (including the initial and final squares) must not be under attack by any opposing piece.
  • Vacant: All the squares between the king's initial and final squares (including the final square), and all of the squares between the rook's initial and final squares (including the final square), must be vacant except for the king and castling rook.

These rules have the following consequences: [...]

  • The king may not be in check before or after castling.

For what seem like "better" rules of castling, see John Kipling Lewis's "Castling in Chess960: An appeal for simplicity" (2005).

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    My quibble with 3.9.2 was indeed based on that (tenuous) grammatical point that the negation of "two pieces" is not "no piece". Incidentally, a pawn is a piece, under the Laws (Article 3 is called "Moves of the pieces"). Interesting point on the transposition moves, although it's equally tenuous. As for David Wheeler, his assertion that "the king may not be in check before or after castling" does not seem to be an immediate logical consequence of the rules he cites. And Lewis' rules are a different can of worms - castling as a concept is a mess really. – Remellion Jan 6 at 13:31
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You have found out all relevant rules, which are the Laws of Chess, including Guideline II. As expected, the wording of the rules seems not to cover that case. In Guideline II, some special cases of castling are considered, but not this one, so Article 3 applies. Article 3 deals with regular chess, and in regular chess your case simply is not possible. Your last resort is 3.9.2, which describes the move of ONE piece. This all is correct.

Now have a look at 3.8.2: "This is a move of the king and either rook of the same colour along the player’s first rank, counting as a single move of the king.“ This should solve your problem.

Additionally, I refer to the preface. Even if the rules would contain a loophole, it is obvious that it is intended that a player may not exhibit his king to an attack. The preface states: "Where cases are not precisely regulated by an Article of the Laws, it should be possible to reach a correct decision by studying analogous situations which are regulated in Laws.“ So no arbiter will allow white to castle in that position.

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    A nice point I missed while reading 3.8. Good catch! (And of course by common sense the castling is illegal - I'm interested in the precise phrasing of the rules, rather than resorting to a catch-all clause like the Preface.) – Remellion Jan 6 at 11:57
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I can’t see the diagrams on mobile, but having read

or the square which it is to occupy

and

the white king ends in check on c1

I would say FIDE covers the scenario: the square which the king is to occupy is attacked if the king would end in check, no?

Or is the issue that the unique 960 arrangement means that the rook previously blocked the check somehow?

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  • The diagram I provided (or the bit of notation just before it) shows that indeed the white rook on b1 blocked black's attack on c1. – Remellion Jan 6 at 11:56

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