FEN Notation
16.1: 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".


16.1.1: 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.


16.1.2: 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.


16.1.3: 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.


16.1.3.1: 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.


16.1.3.2: 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.


16.1.3.3: 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.


16.1.3.4: 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.


16.1.3.5: 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.


16.1.3.6: 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.


16.1.4: 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