Chess rules

The goal

White moves first, then the players take turns (1.2). You win by checkmate: attacking the opponent's king so that they have no legal move (1.4, 5.1.1).

You may never make a move that leaves your own king under attack (3.9.2).

The board sits with a light square in each player's near right-hand corner (2.1).

How the pieces move

The rook moves any distance along its rank or file, the bishop along a diagonal, and the queen along either (3.2–3.4). None of the three may pass over another piece (3.5).

The knight goes to one of the nearest squares not on its own rank, file or diagonal — the far corner of an L (3.6). The no-passing rule covers only the rook, bishop and queen (3.5), so pieces in between do not block a knight.

The king moves one square in any direction (3.8.1).

Pawns, en passant and promotion

A pawn moves one square straight ahead into an empty square, or two on its first move if both are empty (3.7.1, 3.7.2). It captures one square diagonally forward (3.7.3).

En passant: if an enemy pawn has just advanced two squares and stopped beside your pawn, you may capture it as if it had moved one. Only on the very next move (3.7.3.1, 3.7.3.2).

Promotion: a pawn reaching the far rank must become a queen, rook, bishop or knight of its colour. The choice is not limited to pieces already captured (3.7.3.3, 3.7.3.4).

Castling

The king moves two squares towards a rook, and that rook moves to the square the king crossed (3.8.2).

It is lost for good once the king or that rook has moved (3.8.2.1). It is blocked for now while the king is in check, while a square the king crosses or lands on is attacked, or while any piece stands between them (3.8.2.2).

How a game ends

A win: checkmate (5.1.1), the opponent resigns (5.1.2), or the opponent runs out of time (6.9).

A draw: stalemate, when the player to move is not in check and has no legal move (5.2.1); a position where neither side can ever checkmate (5.2.2); or agreement, once both players have made at least one move (5.2.3).

Resigning or running out of time is a draw instead of a loss when the opponent could not checkmate by any series of legal moves (5.1.2, 6.9).

Repetition and the 50-move rule

A player may claim a draw when the same position appears for the third time, or when both sides have made 50 moves without a pawn move or a capture (9.2, 9.3). At five repetitions or 75 moves the draw needs no claim (9.6).

How this site applies them

There is no arbiter to hear a claim, so the game ends in a draw the moment the third repetition or the 50th move is reached.

It recognises three kinds of position where mate is impossible: kings alone; a king and one bishop or knight against a king; and kings with only bishops, all on squares of one colour.

After a resignation or a flag fall, it scores a draw only when the opponent has a lone king. Rarer positions where mate is impossible, such as locked pawn chains, are scored as a loss.