X-FEN

X-FEN is an extension of Forsyth-Edwards Notation (FEN).

The traditional Forsyth-Edwards Notation is not sufficient to represent all possible positions in 8x8 Chess960 (aka Fischer Random Chess) or 10x8 Capablanca random chess (CRC). Consequently, an extension of FEN was needed, with the requirement of being fully backward compatible. X-FEN (formerly FRC-FEN), introduced by Reinhard Scharnagl in 2003, accomplishes this.

X-FEN is based on traditional FEN. It differs only in the way that castling and en passant tags are used. Moreover, 10×8 positions which use princess (knight+bishop) and empress (knight+rook) compound pieces are supported.

Games are translated into PGN format (Portable Game Notation). Each game's starting position must be stored in the PGN for Chess960 and Capablanca random chess(CRC) (but not for traditional chess). Storing the starting position is accomplished with a SetUp tag and an FEN string using the definitions for traditional chess games.

The specification of an en passant target differs slightly from standard Forsyth-Edwards Notation. FEN records the en passant square field as the square just behind a pawn that has made a two-square push forward in the latest move. As such, whenever a pawn makes a two-square move, the "en passant" square is recorded. From the sample game given in Forsyth-Edwards Notation, we see that FEN includes the square e3 as an en passant square after White makes the first move of the game 1. e4. This is somewhat misleading, as no en passant captures can be made by Black from the position.

X-FEN, on the other hand, includes only true en passant squares. That is, X-FEN records a value in the field for an en passant square only if there are one or more enemy pawns on the same rank on an adjacent file. Thus, after 1. e4, the field for the en passant square is left blank, as Black cannot make an en passant capture. However, it is possible that even if an X-FEN records an en passant square, making that capture would be illegal, because after the capture the king of the capturing player would be in check.

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Wikipedia