Section 16.1 from Portable Game Notation.
FEN is “Forsyth-Edwards Notation”; it is a standard for describing chess positions using the ASCII character set.
A single FEN record uses one text line of variable length composed of six data fields. The first four fields of the FEN specification are the same as the first four fields of the EPD specification.
A text file composed exclusively of FEN data records should have a file name with the suffix “.fen”.
FEN is based on a 19th century standard for position recording designed by the Scotsman David Forsyth, a newspaper journalist. The original Forsyth standard has been slightly extended for use with chess software by Steven Edwards with assistance from commentators on the Internet. This new standard, FEN, was first implemented in Edwards’ SAN Kit.
2: Uses for a position notation
Having a standard position notation is particularly important for chess programmers as it allows them to share position databases. For example, there exist standard position notation databases with many of the classical benchmark tests for chessplaying programs, and by using a common position notation format many hours of tedious data entry can be saved. Additionally, a position notation can be useful for page layout programs and for confirming position status for e-mail competition.
Many interesting chess problem sets represented using FEN can be found at the chess.uoknor.edu ftp site in the directory pub/chess/SAN_testsuites.
3: Data fields
FEN specifies the piece placement, the active color, the castling availability, the en passant target square, the halfmove clock, and the fullmove number. These can all fit on a single text line in an easily read format. The length of a FEN position description varies somewhat according to the position. In some cases, the description could be eighty or more characters in length and so may not fit conveniently on some displays. However, these positions aren’t too common.
A FEN description has six fields. Each field is composed only of non-blank printing ASCII characters. Adjacent fields are separated by a single ASCII space character.
3.1: Piece placement data
The first field represents the placement of the pieces on the board. The board contents are specified starting with the eighth rank and ending with the first rank. For each rank, the squares are specified from file a to file h. White pieces are identified by uppercase SAN piece letters (”PNBRQK”) and black pieces are identified by lowercase SAN piece letters (”pnbrqk”). Empty squares are represented by the digits one through eight; the digit used represents the count of contiguous empty squares along a rank. A solidus character “/” is used to separate data of adjacent ranks.
3.2: Active color
The second field represents the active color. A lower case “w” is used if White is to move; a lower case “b” is used if Black is the active player.
3.3: Castling availability
The third field represents castling availability. This indicates potential future castling that may of may not be possible at the moment due to blocking pieces or enemy attacks. If there is no castling availability for either side, the single character symbol “-” is used. Otherwise, a combination of from one to four characters are present. If White has kingside castling availability, the uppercase letter “K” appears. If White has queenside castling availability, the uppercase letter “Q” appears. If Black has kingside castling availability, the lowercase letter “k” appears. If Black has queenside castling availability, then the lowercase letter “q” appears. Those letters which appear will be ordered first uppercase before lowercase and second kingside before queenside. There is no white space between the letters.
3.4: En passant target square
The fourth field is the en passant target square. If there is no en passant target square then the single character symbol “-” appears. If there is an en passant target square then is represented by a lowercase file character immediately followed by a rank digit. Obviously, the rank digit will be “3″ following a white pawn double advance (Black is the active color) or else be the digit “6″ after a black pawn double advance (White being the active color).
An en passant target square is given if and only if the last move was a pawn advance of two squares. Therefore, an en passant target square field may have a square name even if there is no pawn of the opposing side that may immediately execute the en passant capture.
3.5: Halfmove clock
The fifth field is a nonnegative integer representing the halfmove clock. This number is the count of halfmoves (or ply) since the last pawn advance or capturing move. This value is used for the fifty move draw rule.
3.6: Fullmove number
The sixth and last field is a positive integer that gives the fullmove number. This will have the value “1″ for the first move of a game for both White and Black. It is incremented by one immediately after each move by Black.
Here’s the FEN for the starting position:
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq – 0 1
And 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
For two kings on their home squares and a white pawn on e2 (White to move) with thirty eight full moves played with five halfmoves since the last pawn move or capture:
4k3/8/8/8/8/8/4P3/4K3 w – – 5 39