Chess960 FEN disambiguation

Hi all!

How to disambiguate which rook have to castle in fen description of a chess960 intermediate position of a match?

Example of an ambiguous scenario:

starting position: "bbqnnrkr/pppppppp/8/8/8/8/PPPPPPPP/BBQNNRKR w KQkq - 0 1"

after several moves we can have this: "1k6/8/8/8/8/8/8/3R1RK1 w Q"

but not knowing the initial position, we cannot know which rook have to castle.

Is there a particular notation for these scenarios?

The whole idea of KQkq designation in the fen doesn't work for Chess960 yet this website still uses it (but not sure about v3). I think your problem is solved with this:

"The solution implemented by chess engines like Shredder and Fritz is to use the letters of the columns on which the rooks began the game. This scheme is sometimes called Shredder-FEN. For the traditional setup, Shredder-FEN would use HAha instead of KQkq."

Taken from: https://en.wikipedia.org/wiki/Chess960#Coding_games_and_positions

In your example above the fen would be:

"bbqnnrkr/pppppppp/8/8/8/8/PPPPPPPP/BBQNNRKR w HFhf - 0 1"

So it keeps a record of where the rooks started embedded in the fen.

Hope that helps

So down the track here is a fen during the game with the improved format:

bbqr2kr/pp1ppppp/4nn2/2p5/2P5/2Q1NN2/PP1PPPPP/BB3RKR w HFh - 6 5

It is saying that white can still castle "kingside" and "queenside" but black only kingside (HFh.)

Note that kingside and queenside are meaningless in chess960 but we still use it by habit. Chess960 lingual people say instead:

Queenside = A side

Kingside = H side

However I personally don't like that and prefer this:

Queenside = C side

Kingside = G side

because that not only tells you what side of the board castling is refering to but also the position of the king after the castling has taken place. So it has more information embedded in it. Note though, nobody I know of uses that terminology, almost everyone A-H side or just kingside, queenside.

There are actually two standards for Chess960 FEN: Shredder FEN and X-FEN. The former was described above, and always uses the file letters to indicate castling rights. The disadvantage of this is that the FEN for a Chess960 position would be different from the FEN of exactly the same position in orthodox Chess.

For better compatibility with orthodox Chess, X-FEN uses the convention that K and Q must be used to indicate the outer-most Rook on H-side and A-side, respectively. The file letters are only used when there are multiple Rooks on that side, and it is the inner-most one that can do the castling.

In Seirawan Chess (S-Chess) the situation is even more complex, because not only the virginity of Rooks and King can be relevant for castling, but also of all the back-rank pieces, for gating. So the FEN there has a general virginity field, instead of the castling field, mentioning all files that contain pieces that have not moved yet. For backward compatibility, virginity that does not have any consequences (because there are no longer any pieces in hand that could be gated, and the piece cannot castle anymore, or never could), are not written. And AE must be contracted to Q, and EH to K. So the initial position would have KGFDCBQkgfdcbq.

great answers! thank you