Shredder-FEN

FEN, Forsyth-Edwards Notation, is a text string describing a chess position. One field records castling rights, meaning whether each side still retains the rule-based possibility of making the special move involving its king and a rook.

In standard chess, KQkq distinguishes kingside and queenside rights. Chess960 is a variant that rearranges the back-rank pieces and can place the king and rooks on files different from their standard ones. KQkq may therefore fail to identify precisely enough which rook holds each right.

Shredder-FEN removes that ambiguity by writing the file letter of the relevant rook. A file is a vertical column named from a through h. Uppercase letters represent White's rooks and lowercase letters represent Black's. In the standard starting position, HAha represents the h-file and a-file rooks for both sides.

The field records rights, not whether castling can be played immediately. Blocking pieces, attacked squares, or other conditions may prevent the move in the current position even while the right remains. A hyphen means that no castling rights remain.

X-FEN is another convention extending FEN for Chess960. Because the two encodings are not identical, writing and reading software must agree on which one is being used.