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Forsyth-Edwards Notation (FEN) describes a Chess Position. It is an one-line ASCII-string. FEN is based on a system created by Scotsman David Forsyth in the 19th century. Steven Edwards specified the FEN standard for computer chess applications as part of the Portable Game Notation ^[1] .

FEN Syntax

One FEN string or record consists of six fields separated by a space character. The first four fields of the FEN specification are the same as the first four fields of the EPD specification.

Terminal and nonterminal symbols of a variant of BNF below are embedded in ' ' resp. < >.

<FEN> ::=  <Piece Placement>
       ' ' <Side to move>
       ' ' <Castling ability>
       ' ' <En passant target square>
       ' ' <Halfmove clock>
       ' ' <Fullmove counter>

Piece Placement

The Piece Placement is determined rank by rank in big-endian order, that is starting at the 8th rank down to the first rank. Each rank is separated by the terminal symbol '/' (slash). One rank, scans piece placement in little-endian file-order from the A to H.

A decimal digit counts consecutive empty squares, the pieces are identified by a single letter from standard English names for chess pieces as used in the Algebraic chess notation. Uppercase letters are for white pieces, lowercase letters for black pieces.

<Piece Placement> ::= <rank8>'/'<rank7>'/'<rank6>'/'<rank5>'/'<rank4>'/'<rank3>'/'<rank2>'/'<rank1>
<ranki>       ::= [<digit17>]<piece> {[<digit17>]<piece>} [<digit17>] | '8'
<piece>       ::= <white Piece> | <black Piece>
<digit17>     ::= '1' | '2' | '3' | '4' | '5' | '6' | '7'
<white Piece> ::= 'P' | 'N' | 'B' | 'R' | 'Q' | 'K'
<black Piece> ::= 'p' | 'n' | 'b' | 'r' | 'q' | 'k'

Side to move

Side to move is one lowercase letter for either White ('w') or Black ('b').

<Side to move> ::= {'w' | 'b'}

Castling ability

If neither side can castle, the symbol '-' is used, otherwise each of four individual castling rights for king and queen castling for both sides are indicated by a sequence of one to four letters.

<Castling ability> ::= '-' | ['K'] ['Q'] ['k'] ['q'] (1..4)

En passant target square

The en passant target square is specified after a double push of a pawn, no matter whether an en passant capture is really possible or not ^{[2] [3] [4]} . Other moves than double pawn pushes imply the symbol '-' for this FEN field.

<En passant target square> ::= '-' | <epsquare>
<epsquare>   ::= <fileLetter> <eprank>
<fileLetter> ::= 'a' | 'b' | 'c' | 'd' | 'e' | 'f' | 'g' | 'h'
<eprank>     ::= '3' | '6'

Halfmove Clock

The halfmove clock specifies a decimal number of half moves with respect to the 50 move draw rule. It is reset to zero after a capture or a pawn move and incremented otherwise.

<Halfmove Clock> ::= <digit> {<digit>}
<digit> ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'

Fullmove counter

The number of the full moves in a game. It starts at 1, and is incremented after each Black's move.

<Fullmove counter> ::= <digit19> {<digit>}
<digit19> ::= '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
<digit>   ::= '0' | <digit19>

Samples

FEN strings of Starting Position and after 1.e4 c5 2.Nf3:
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1
rnbqkbnr/pp1ppppp/8/2p5/4P3/8/PPPP1PPP/RNBQKBNR w KQkq c6 0 2
rnbqkbnr/pp1ppppp/8/2p5/4P3/5N2/PPPP1PPP/RNBQKB1R b KQkq - 1 2

Extensions for Chess Variants

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See also

• Extended Position Description (EPD)
• Forsyth-Edwards Expanded Notation (FEEN)
• Portable Game Notation (PGN)

Forum Posts

2000 ...

• File name extensions by Leen Ammeraal, CCC, November 14, 2000
• Making positions in eps by Renze Steenhuisen, CCC, October 27, 2003 » Fen2eps

2005 ...

• contradicting FEN and SMK-FEN by Reinhard Scharnagl, CCC, August 04, 2005
• fen to fen functions by Uri Blass, CCC, May 21, 2007

2010 ...

• where FEN is not consistent by Reinhard Scharnagl, CCC, January 06, 2010
• FEN string by colin, CCC, January 30, 2011
• No more pseudolegal en passant target foolishness by Steven Edwards, CCC, February 27, 2011
• What's wrong with this EPD? by Jouni Uski, CCC, March 20, 2011
• Question about Shredder FEN and X-FEN by Harm Geert Muller, CCC, April 22, 2012
• Re: Causes for inconsistent benchmark signatures by Evert Glebbeek, CCC, March 27, 2013 » En passant
• The maximum character length of a FEN string by Steven Edwards, CCC, August 24, 2013
• Is 79 maximal? by Louis Zulli, CCC, July 29, 2014
• PGN to FEN (with Evaluation)? by Steve Maughan, CCC, December 28, 2014 » Portable Game Notation, Python

2015 ...

• Binary FEN by J. Wesley Cleveland, CCC, July 24, 2015
• Any tool to convert FEN strings to diagrams? by Ted Wong, CCC, February 15, 2016
• FEN - Flipper for Windows by Matthias Gemuh, CCC, May 17, 2017 » Color Flipping, EPD

External Links

• Forsyth-Edwards Notation from Wikipedia
• Chess Programming - Chess Board Implementation : A FEN parser by Thomas Petzke
• FEN Database by Mathieu Pagé
• Gilith - Chess Diagram Maker by Joe Leslie-Hurd
  fen2img Chess Diagram Maker by Joe Leslie-Hurd
• Chess Diagram Generator
• Chessforeva: 3D chess diagram from FEN » 3D Graphics Board
• Fen2eps by Dirk Baechle ^[5]

References

1. ^ Standard: Portable Game Notation Specification and Implementation Guide 16.1: FEN by Steven Edwards
2. ^ Re: Arasan test suite update by Steven Edwards, CCC, September 19, 2008
3. ^ where FEN is not consistent by Reinhard Scharnagl, CCC, January 06, 2010
4. ^ No more pseudolegal en passant target foolishness by Steven Edwards, CCC, February 27, 2011
5. ^ Encapsulated PostScript from Wikipedia

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