Forsyth–Edwards Notation (FEN) is a standard notation for describing a particular board position of a chess game. The purpose of FEN is to provide all the necessary information to restart a game from a particular position.
FEN is based on a system developed by the Scottish newspaper journalist, David Forsyth. Forsyth's system became popular in the 19th century; Steven J. Edwards extended it to support use by computers. FEN is an integral part of the Portable Game Notation for chess games, since FEN is used to define initial positions other than the standard one. FEN does not represent sufficient information to decide on a draw by threefold repetition; for that, a different format such as Extended Position Description is needed.
A FEN "record" defines a particular game position, all in one text line and using only the ASCII character set. A text file with only FEN data records should have the file extension ".fen".
A FEN record contains six fields. The separator between fields is a space. The fields are:
- Piece placement (from white's perspective). Each rank is described, starting with rank 8 and ending with rank 1; within each rank, the contents of each square are described from file "a" through file "h". Following the Standard Algebraic Notation (SAN), each piece is identified by a single letter taken from the standard English names (pawn = "P", knight = "N", bishop = "B", rook = "R", queen = "Q" and king = "K"). White pieces are designated using upper-case letters ("PNBRQK") while black pieces use lowercase ("pnbrqk"). Blank squares are noted using digits 1 through 8 (the number of blank squares), and "/" separates ranks.
- Active color. "w" means white moves next, "b" means black.
- Castling availability. If neither side can castle, this is "-". Otherwise, this has one or more letters: "K" (White can castle kingside), "Q" (White can castle queenside), "k" (Black can castle kingside), and/or "q" (Black can castle queenside).
- En passant target square in algebraic notation. If there's no en passant target square, this is "-". If a pawn has just made a two-square move, this is the position "behind" the pawn. This is recorded regardless of whether there is a pawn in position to make an en passant capture.
- Halfmove clock: This is the number of halfmoves since the last pawn advance or capture. This is used to determine if a draw can be claimed under the fifty-move rule.
- Fullmove number: The number of the full move. It starts at 1, and is incremented after Black's move.
Here is the FEN for the starting position:
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
Here is the FEN after the move 1. e4:
rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1
And then after 1. ... c5:
rnbqkbnr/pp1ppppp/8/2p5/4P3/8/PPPP1PPP/RNBQKBNR w KQkq c6 0 2
And then after 2. Nf3:
rnbqkbnr/pp1ppppp/8/2p5/4P3/5N2/PPPP1PPP/RNBQKB1R b KQkq - 1 2
FEN adjustment for chess variants like Chess960 
FEN notation is critical for recording games in chess variants such as Chess960 (also known as Fischer Random Chess), where the initial position is not necessarily the traditional initial position. However, the FEN castling availability encoding (KQkq) is inadequate for positions in which there are two Rooks on the same side of the King on the back rank. It is ambiguous which Rook is still available for castling without knowing their initial positions.
The solution implemented by chess engines like Shredder and Fritz_9 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 AHah instead of KQkq.
Another solution is offered by X-FEN, which offers more backward compatibility than does Shredder-FEN, but at the cost of more complexity.