Forsyth-Edwards Notation

FEN stands for Forsyth-Edwards Notation. It turns a position into one line of text so that people and programs can save, copy, and rebuild it. It does not tell the story of the game or show how the position arose. It records the board plus rights and counters that can affect legal play.

The six fields

1. Piece placement: reads the board from rank eight down to rank one. Uppercase letters are White's pieces, lowercase letters are Black's pieces, and digits count consecutive empty squares.
2. Active color: w means White to move and b means Black to move.
3. Castling availability: K, Q, k, and q show which castling rights remain. A hyphen means none remain.
4. En passant target: records the square crossed by a pawn that has just advanced two squares. The standard records it even when no immediate legal en passant capture exists.
5. Halfmove clock: counts individual turns since the last capture or pawn move and helps apply the fifty-move draw rule.
6. Fullmove number: starts at 1 and increases after each Black move.

FEN is used to share puzzles, resume analysis, and give positions to a chess engine. It differs from PGN, which normally records a game and its moves, and from EPD, which can attach structured instructions or attributes to a position.

A string can follow FEN syntax yet describe a position that could never have arisen through legal play. When that distinction matters, reliable software should validate both the text format and the chess position itself.