Forsyth-Edwards Notation Explained
A FEN string contains six fields or sections. The separator between fields is a space. The fields are:
Field 1
Piece placement (from white’s perspective):
Each rank of the chessboard is described, starting with rank 8 and ending with rank 1; within each rank, the contents of each square are described from the a-file to the h-file.
Following the Standard Algebraic Notation (SAN), each piece is identified by a single letter taken from the standard English names (pawn = “P”, knight = “N”, bishop = “B”, rook = “R”, queen = “Q” and king = “K”).
White pieces are designated using upper-case letters (“PNBRQK”)
Black pieces use lowercase (“pnbrqk”).
Empty squares are noted using digits 1 through 8 (the number of empty squares), and “/” separates ranks.
Field 2
Active colour:
“w” means its White’s turn to move.
“b” means its Black’s turn to move.
Field 3
Castling availability:
If neither side can castle, this is “-“. Otherwise, this has one or more letters: “K” (White can castle kingside), “Q” (White can castle queenside), “k” (Black can castle kingside), and/or “q” (Black can castle queenside).
Field 4
If an en-passant capture can be made, this shows the square that the capturing pawn will end up on. If no en passant move can be made then this field is just “-“. This field is active regardless of whether or not there is a pawn in a position to make an en passant capture.
Field 5
Halfmove clock:
This is the 50-move rule counter. It starts at ‘0‘. As soon as a move is played that is NEITHER a pawn move NOR a capture, this counter starts counting. If a pawn move and/or capture is subsequently played, the counter resets to ‘0‘.
Field 6
Fullmove counter:
This counts the number of complete moves played. At the start of a game, it is set to 1, where it is waiting for move 1 to be played. White makes a move and, as soon as black replies, it increments to 2, showing that move 1 has been completed and it is now waiting for move 2 to be played.
Example 1:
Here is the FEN string for the start of a game of chess.
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq – 0 1
Field 1:- rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR
Field 2:- w
Field 3:- KQkq
Field 4:- ‘-’
Field 5:- 0
Field 6:- 1
Field 1:-
Going rank by rank starting with the 8th rank we have
rnbqkbnr – These are Black’s pieces:- rook, knight, bishop, queen, king, bishop, knight, rook
pppppppp – These are Black’s pawns
8/8/8/8/ – These are the 4, empty ranks of 8 squares each.
PPPPPPPP – These are White’s pawns.
RNBQKBNR – These are White’s pieces:- Rook, Knight, Bishop, Queen, King, Bishop, Knight, Rook
Field 2:-
w – It is White’s turn to play
Field 3:-
K – White can castle kingside at some time.
Q – White can castle queenside at some time
k – Black can castle kingside at some time.
q- Black can castle queenside at some time.
Field 4
‘-’ – No en-passant move can be played in the position
Field 5
‘0‘ – This is the 50-move rule counter. No pawn moves or captures have yet been played, so this is set to 0.
Field 6
‘1‘ – This is the move counter. It starts at 1, showing that it is waiting for the first, complete move to be played. It is increased by one after each of Black’s moves.
Example 2:
rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1
Here, white has opened the game with 1.e4. Field 1 shows this move. The part of the field that looks like /4P3/ shows that, along the 4th rank, there are 4 empty squares, a white pawn, then 3 empty squares. If this pawn could be taken en-passant, then e3 would be the square that the capturing pawn would end up on, as shown by the ‘e3‘ in field 4. Remember that this field is active whether or not there is a black pawn that can actually make an en-passant capture in the position.
Black now replies with the Sicilian Defence and plays 1…c5, producing the following FEN string:-
rnbqkbnr/pp1ppppp/8/2p5/4P3/8/PPPP1PPP/RNBQKBNR w KQkq c6 0 2
Field 1 shows that, along the 5th rank there are now 2 empty squares, a black pawn, then 5 empty squares.
Field 4 shows that, if this pawn could be taken en-passant, the square that the capturing pawn would end up on is c6.
Field 5 shows that, since a pawn move has been made, so there is no need to start the 50-move rule count. This number is still 0.
Field 6 shows that one, complete move has been played, and we are now waiting for move 2.
White now continues with 2.Nf3 giving the following FEN string,
rnbqkbnr/pp1ppppp/8/2p5/4P3/5N2/PPPP1PPP/RNBQKB1R b KQkq – 1 2
Field 1 shows that, in addition to the previous moves, there are now 5 empty squares, a white knight, then 2 empty squares along the third rank.
Field 4 shows that no en-passant capture is now possible, hence, ‘-’
Field 5 shows that, since white’s last move was not a pawn move or capture, the 50-move rule count has started.
So there we have it. Not too difficult to get your head around.
