Opening classification uses modified FEN notation, i.e. the halfmove and
fullmove numbers are dropped since these are irrelvant for opening positions.

---

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