FEN explained: the text format behind every chess position

Every analysis board, puzzle and engine request needs a way to describe a position. FEN, the Forsyth-Edwards Notation, is the standard. It is compact, exact and easier to read than it looks.

The six fields

A FEN string has six space-separated fields. Here is the starting position:

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

1. Piece placement: the board from rank 8 to rank 1, with slashes between ranks. Lowercase letters are Black, uppercase are White, and digits are empty squares.
2. Side to move: w or b.
3. Castling rights: KQkq means both sides can castle both ways. A missing letter means that right has been lost.
4. En passant target: the square behind a pawn that just moved two squares, or -.
5. Halfmove clock: moves since the last capture or pawn move. It is used for the fifty-move rule.
6. Fullmove number: starts at 1 and increments after Black's move.

Reading the board

The first rank in the string is rank 8, not rank 1. 8 means eight empty squares. p is a black pawn, P a white pawn, n a knight, b a bishop, r a rook, q a queen and k a king.

So 4k3/8/8/8/8/8/4P3/4K3 w - - 0 1 is a white king on e1, a white pawn on e2 and a black king on e8.

Common mistakes

• Writing the board from rank 1 upward. FEN always starts at rank 8.
• Forgetting the side-to-move field. A board without it is incomplete.
• Using a FEN with impossible castling rights or en passant squares. Engines may reject it or analyse a different position.
• Ignoring the halfmove clock when checking the fifty-move rule.

Why FEN matters

FEN is how you share a position with an engine, a friend or a study tool. It is also how ChessUrge stores positions for analysis and generated puzzles. If a position looks wrong after pasting a FEN, check the side to move first; it is the most common error.

You can paste a FEN into the analysis board to test it. If you want the move format that goes with it, read PGN explained.