Forsyth-Edwards Notation

1. FEN

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".


2. History

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.


3. 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.


4. 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.


5. 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.


6. 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.


7. 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.


8. 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.


9. 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.


10. 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.


11. Examples

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