FEN Editor and Validator

White to move

–

0 moves

About FEN

FEN stands for Forsyth-Edwards Notation. It is a single line of text that describes a complete chess position: where every piece stands, whose turn it is, and the rules state that a game needs in order to continue correctly.

The six fields

The position after 1. e4, field by field

rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR1b2KQkq3-40516

Piece placement: Eight ranks separated by slashes. Uppercase letters are White pieces, lowercase are Black, and digits count empty squares.
Side to move: w means White is to move; b means Black is to move.
Castling rights: K and Q mean White may still castle kingside or queenside; k and q mean the same for Black. A dash means neither side may castle.
En passant target: The square behind a pawn that has just advanced two squares, or a dash when en passant is not available.
Halfmove clock: How many halfmoves have passed since the last capture or pawn move. It drives the fifty-move rule.
Fullmove number: The number of completed moves. It increases after Black moves.

Common positions

The starting position.

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

Open on the board

After 1. e4 — a White pawn on e4 and Black to move.
```
rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1
```
Open on the board
Kings and rooks on their home squares, with full castling rights for both sides.
```
r3k2r/pppppppp/8/8/8/8/PPPPPPPP/R3K2R w KQkq - 0 1
```
Open on the board

Why the board rejects some FEN strings

A FEN can be syntactically valid and still describe an impossible position — two kings, a pawn on the first rank, or a side that is already in check while the opponent is to move. Aback Chess validates before loading, and reports which field is wrong rather than guessing at a correction.

Next step

Once a position is set, open it on the analysis board to get the engine evaluation, the best continuation, and the evaluation graph as you explore.