Legal move

A move is legal when the rules allow it in the exact position on the board. It is not enough for the piece to follow its usual movement pattern: blocked paths, the destination square, special rules, and the safety of the player's own king all matter.

• The piece must move or capture according to its rules, without passing through pieces unless it is a knight.
• The destination square cannot contain a piece of the same color.
• A castling move or an en passant capture must satisfy its extra conditions.
• After the move, the player's own king cannot remain under attack.

When the king is in check, only moves that remove the attack are legal. A legal move can still be a poor decision, lose a piece, or lead to defeat; legal describes compliance with the rules, not strategic quality.