Illegal move

A move is illegal when it fails any rule that applies to the position. The problem may be the piece's movement, a blocked path, an unmet special condition, or the fact that the player's own king remains under attack.

Common cases

• Moving a rook diagonally or a pawn backward.
• Making a bishop, rook, or queen pass through another piece.
• Moving the king onto an attacked square or failing to answer an existing check.
• Attempting castling after the king has moved, through an attacked square, or with pieces in between.
• Trying an en passant capture after its one-move opportunity has expired.

In a competition played in person with physical pieces, the arbiter, the official who applies the rules, follows the correction and penalty procedure set for that event and speed of play. On a digital platform, the interface usually rejects the attempt before it becomes a played move. A poor move that still follows every rule is not illegal; it is simply a mistake.