FEN (Forsyth-Edwards Notation) is a standard notation for describing a chess position. A single FEN string encodes the entire board layout, whose turn it is, castling rights, en passant targets, and move counters. It's widely used by chess engines, databases, and online platforms to save and share positions.

A FEN string has 6 space-separated fields: Piece placement (8 ranks separated by slashes, lowercase=black, uppercase=white, digits=empty squares); Active color (w/b); Castling rights (KQkq or -); En passant target square (e.g., e3 or -); Halfmove clock; Fullmove number.

w = White to move. KQkq = Both sides have full castling rights (White: Kingside & Queenside, Black: Kingside & Queenside). - = No en passant target. 0 = Halfmove clock (for 50-move rule). 1 = Fullmove number (increments after Black's move).

If a pawn has just moved two squares forward, the en passant target square is recorded in the FEN (e.g., "e3" means a pawn on e2 moved to e4, and the opponent can capture en passant on e3). If no en passant is possible, the field contains a dash "-".

K = White can castle kingside, Q = White can castle queenside, k = Black can castle kingside, q = Black can castle queenside. A dash "-" means no castling is possible. Rights are lost when the king or rook moves.

You can click pieces to select and move them around the board, use the piece palette to place new pieces, delete pieces with the eraser tool, load positions via FEN strings, flip the board perspective, and analyze material balance. It's perfect for studying positions, setting up puzzles, or sharing board states with others.