What the FEN fields mean
The line under the board is a complete description of the position. Reading left to right: the piece placement rank by rank starting from Black’s back rank, the side to move (w or b), the remaining castling rights, the en passant target square, and the two move counters.
Uppercase letters are White’s pieces, lowercase are Black’s, and a digit means that many empty squares in a row. That is the whole format — it is designed to be short enough to paste anywhere.
Legality is checked as you build
A position that could never occur in a game will be rejected by engines and databases, so the editor validates as you go. The common traps are a side with no king, a pawn on the back rank, and castling rights left ticked after you have moved the rook off h1.
Where this is useful
- Reconstructing a position from a book, a diagram, or a photo of a board.
- Building a study or endgame position to test an idea, then asking the engine what it thinks.
- Setting up the exact moment a game went wrong, so you can look at the alternatives you did not play.
If the position came from your own game
Rebuilding it by hand is the slow route. Import the game from Chess.com or Lichess instead and every position is already there, with every move graded — which usually shows you that the moment worth studying was several moves before the one you remembered.
Frequently asked questions
How do I set up a position?
Pick a piece from the palette, then click the squares you want it on. Use Erase to remove a piece, Clear board to start from nothing, or Starting position to reset. The FEN at the bottom updates as you go.
Why does it say the position is not legal?
A FEN has to describe a position a real game could reach. The usual causes are a missing king on either side, a pawn sitting on the first or eighth rank, or castling rights ticked for a rook that is not on its starting square. Fix the board and the warning clears.
What is the FEN for?
FEN is the standard one-line format for a chess position. Every serious chess site and engine accepts it, so you can paste the output into an analysis board, a database search, a puzzle editor, or a forum post.
Can I analyse the position I built?
Yes — once the position is legal, the "Find the best move here" button hands the FEN to our engine tool, which runs Stockfish 18 in your browser and returns the strongest move with its evaluation.