X-FEN

X-FEN, formerly called FRC-FEN, extends FEN, or Forsyth-Edwards Notation. FEN is a compact string describing a position: piece placement, side to move, castling rights, a possible en passant capture, and move counters. X-FEN keeps that model but adapts selected fields for Chess960, a variant with 960 legal starting arrangements of the back-rank pieces.

The main changes

• In the castling field, K and Q normally refer to the outer rook on the king side or queen side. If an inner rook owns the right, its file letter is used, uppercase for White and lowercase for Black.
• The en passant target is written only when an opposing pawn on an adjacent file could geometrically make that capture. The capture can still be illegal if it would expose that pawn's own king to check.
• Ordinary positions that need no extension retain an encoding compatible with traditional FEN.

X-FEN and Shredder-FEN address the same interoperability problem, but they do not always encode castling identically. Shredder-FEN consistently identifies the relevant rooks by file letters; X-FEN keeps K and Q when they are sufficient and uses file letters when an inner rook must be distinguished.

X-FEN is a snapshot of one position, not a complete game record. It carries enough state to resume from that point, but it does not by itself preserve every earlier move.