View source for Forsyth–Edwards Notation

{{short description|Notation for describing a chess game position}}

'''Forsyth–Edwards Notation''' ('''FEN''') is a standard [[Chess notation|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 [[Scotland|Scottish]] newspaper journalist [[David Forsyth (chess player)|David Forsyth]]. His system became popular in the 19th century, then [[Steven J. Edwards]] extended it to support its use by computers. FEN is defined{{refn|Section "16.1: FEN" in "Portable Game Notation Specification and Implementation Guide"<ref name="pgn_spec" />}} in the "Portable Game Notation Specification and Implementation Guide".<ref name="pgn_spec" /> In the  [[Portable Game Notation]] for chess games, FEN is used to define initial positions other than the standard one.{{refn|Sections "9.7.1: Tag: SetUp" and "9.7.2: Tag: FEN" in "Portable Game Notation Specification and Implementation Guide"<ref name="pgn_spec" />}} FEN does not provide sufficient information to decide whether a [[draw (chess)|draw]] by [[threefold repetition]] may be legally claimed or a [[draw offer]] may be accepted; for that, a different format such as [[Extended Position Description]] is needed.

==Definition==
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 use the [[filename extension]] <code>.fen</code>.{{refn|Last paragraph in section "16.1: FEN" in "Portable Game Notation Specification and Implementation Guide"<ref name="pgn_spec" />}}

A FEN record contains six fields, each separated by a space. The fields are as follows:{{refn|Sections 16.1.3.1 to 16.1.3.6: in "Portable Game Notation Specification and Implementation Guide"<ref name="pgn_spec">{{Cite web|date=12 March 1994|title=Standard: Portable Game Notation Specification and Implementation Guide|url=https://archive.org/download/pgn-standard-1994-03-12/PGN_standard_1994-03-12.txt|access-date=25 July 2020|website=Internet Archive}} The page linking to the document is [https://archive.org/details/pgn-standard-1994-03-12 here].</ref>}}
# Piece placement data: Each {{chessgloss|rank}} is described, starting with rank 8 and ending with rank 1, with a "/" between each one; within each rank, the contents of the squares are described in order from the a-{{chessgloss|file}} to the h-file. Each piece is identified by a single letter taken from the standard English names in [[Algebraic notation (chess)|algebraic notation]] (pawn = "P", knight = "N", bishop = "B", rook = "R", queen = "Q" and king = "K"). White pieces are designated using uppercase letters ("PNBRQK"), while black pieces use lowercase letters ("pnbrqk"). A set of one or more consecutive empty squares within a rank is denoted by a digit from "1" to "8", corresponding to the number of squares.
# Active color: "w" means that White is to move; "b" means that Black is to move.
# [[Castling]] availability: If neither side has the ability to castle, this field uses the character "-". Otherwise, this field contains one or more letters: "K" if White can castle kingside, "Q" if White can castle queenside, "k" if Black can castle kingside, and "q" if Black can castle queenside. A situation that temporarily prevents castling does not prevent the use of this notation.
# ''[[En passant]]'' target square: This is a square over which a pawn has just passed while moving two squares; it is given in algebraic notation. If there is no ''en passant'' target square, this field uses the character "-". This is recorded regardless of whether there is a pawn in position to capture ''en passant''.{{refn|Section "16.2.3.4: En passant target square" in "Portable Game Notation Specification and Implementation Guide"<ref name="pgn_spec" />}} An updated version of the spec has since made it so the target square is recorded only if a legal ''en passant'' capture is possible, but the old version of the standard is the one most commonly used.<ref>{{cite web |url=http://www.talkchess.com/forum3/viewtopic.php?p=588125&sid=ec27c00639b0e2ab009cc3cdb6072e42#p588125 |title=About that en passant target square |website=TalkChess.com |date=September 16, 2014 |archive-url=https://web.archive.org/web/20220702113904/http://www.talkchess.com/forum3/viewtopic.php?p=588125&sid=ec27c00639b0e2ab009cc3cdb6072e42 |archive-date=2022-07-02 |url-status=live}}</ref><ref>{{cite web |url=https://github.com/fsmosca/PGN-Standard/blob/master/PGN-Standard.txt |title=Portable Game Notation Specification and Implementation Guide |website=GitHub |date=2020-06-03 |archive-url=https://web.archive.org/web/20220702111242/https://github.com/fsmosca/PGN-Standard/blob/master/PGN-Standard.txt |archive-date=2022-07-02 |url-status=live}}</ref>
# Halfmove clock: The number of halfmoves since the last capture or pawn advance, used for the [[fifty-move rule]].{{refn|Section "16.1.3.5: Halfmove clock" states "This value is used for the fifty move draw rule."<ref name="pgn_spec" />}}
# Fullmove number: The number of the full moves. It starts at 1 and is incremented after Black's move.

==Examples==
The following example is from the FEN specification:{{refn|Section "16.1.4: Examples" in "Portable Game Notation Specification and Implementation Guide"<ref name="pgn_spec" />}}

Here is the FEN for the starting position:
<pre>
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
</pre>

And after the move 1.e4:
<pre>
rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1
</pre>

And then after 1...c5:
<pre>
rnbqkbnr/pp1ppppp/8/2p5/4P3/8/PPPP1PPP/RNBQKBNR w KQkq c6 0 2
</pre>

And then after 2.Nf3:
<pre>
rnbqkbnr/pp1ppppp/8/2p5/4P3/5N2/PPPP1PPP/RNBQKB1R b KQkq - 1 2
</pre>

==FEN adjustment for chess variants like Chess960 ==
FEN is crucial 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 when both rooks are on the same side of the king on the back rank (as a result of one rook having moved, or in a form of randomised chess that allows it in a starting position): if either rook is still available for castling, it would be ambiguous which rook this is 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.<ref>{{cite web |url=https://www.chessprogramming.org/Forsyth-Edwards_Notation#Shredder-FEN |title=Shredder-FEN |website=www.chessprogramming.org |date=April 5, 2022}}</ref> 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 Shredder-FEN does, but at the cost of more complexity.

==See also==
*[[Portable Game Notation]]
*[[Extended Position Description]]

==References ==
{{Reflist}}

==External links ==
*[https://www.chessclub.com/help/PGN-spec Portable Game Notation Specification and Implementation Guide, Steven J. Edwards]

{{DEFAULTSORT:Forsyth-Edwards Notation}}
[[Category:Computer chess]]
[[Category:Chess notation]]
[[Category:Computer file formats]]