What is FEN?

FEN stands for Forsyth–Edwards Notation. It is a short line of text describing one position: which pieces are on the board, whose turn it is, and a few rules that depend on what happened earlier. It is useful for sharing a puzzle or starting a new game from a particular setup. It does not contain the moves that led there; use PGN for a whole game.

The normal starting position is:

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

The six space-separated parts mean:

1. Piece placement. The eight slash-separated rows run from Black’s back rank to White’s back rank. Letters name pieces; capital letters are White, lowercase letters are Black. Numbers count empty squares.
2. Side to move. w means White moves next; b means Black.
3. Castling rights. KQkq says both sides may still castle on either side, if the other conditions are met. - means no castling rights. Chess960 positions can use rook-file letters.
4. En passant target. A square where an en passant capture is available, or - if none. En passant is a special pawn capture immediately after an opposing pawn moves two squares.
5. Halfmove clock. The number of half-moves since the last pawn move or capture; it matters for the fifty-move draw rule.
6. Fullmove number. The current move number, beginning at 1 and increasing after Black moves.

You do not need to write FEN by hand. In Ironwood, use Tools → Position Editor to arrange the board and copy its FEN. To play from it, choose Play → New Play → Local → Starting position: Custom FEN, paste the complete FEN, and check that Ironwood says the position is valid. You can also use Edit board / Import screenshot from that dialog. See Using the Position Editor for a walkthrough.

Be careful with the non-board fields: a picture alone cannot tell whose turn it is or whether castling and en passant are still legal.

If you copied a FEN from a puzzle website, follow Import a chess puzzle from another site to set it up and explore the moves in Ironwood.