Illegal move - chess term
Illegal move
Definition
An illegal move is any action that violates the rules of chess. Typical examples include leaving your king in check, moving a piece in a way it cannot legally move, castling through or out of check, claiming en passant when it is not available, or promoting a pawn incorrectly. In formal play, an illegal move is not allowed to stand: the position must be restored to the last legal position and a penalty is applied according to the competition rules.
How the term is used
Players, commentators, and arbiters use “illegal move” to identify a move that cannot be played under the Laws of Chess. In over-the-board (OTB) tournaments, if a player completes an illegal move and presses the clock, the opponent can stop the clocks and call the arbiter. Online, the interface typically prevents illegal moves entirely, so the term is more often used descriptively (e.g., “Castling here would be illegal because the king is in check”).
Rules and penalties (FIDE-oriented overview)
- Correction: If an illegal move has been completed (the player released the piece and pressed the clock), the arbiter reinstates the position to the last legal one and requires the player to make a legal move with the touched piece if possible (touch-move still applies).
- Time penalties:
- Standard and Rapid (with adequate supervision): Typically, the opponent receives extra time (commonly 2 minutes). After a second illegal move by the same player, that player loses the game.
- Blitz: Often similar (time added to the opponent; second illegal move loses). In some blitz/rapid events without adequate supervision, a single illegal move may forfeit the game immediately. Always check event regulations.
- Claim timing: If your opponent makes an illegal move and you then make your own move, you generally forfeit the right to claim for that illegal move.
- The king is never “captured”: Any move that would “take the king” is illegal; instead, checks must be parried legally.
Common types of illegal moves
- Leaving the king in check or moving into check (e.g., 1...Ke7 when a rook on e1 controls the e-file).
- Ignoring a check (playing a non-check-parrying move when your king is attacked).
- Illegal castling:
- Castling while in check, through a checked square, or into check.
- Castling after the king or the relevant rook has moved earlier.
- Castling with pieces in between the king and rook.
- Incorrect promotion (e.g., not replacing the pawn with a piece of the same color on the promotion square; using an inverted rook “as a queen” is not a queen under FIDE rules).
- Invalid en passant (only legal immediately after a two-square pawn advance by the opponent, and only by a pawn on the fifth rank capturing as if that pawn had advanced one square).
- Moving a piece in a way it cannot move (e.g., sliding a knight like a bishop).
- Using two hands to execute a move (e.g., two-handed castling) can be penalized as an illegal action/irregularity under tournament rules.
Strategic and practical significance
Illegal moves matter most in time trouble. Knowing the rules precisely can prevent disastrous penalties. Strong practical players also know how to claim properly and calmly: stop the clock, summon the arbiter, and explain the sequence. While you should never “play for” an opponent’s illegal move, accurate awareness can save you both time and half-points in tense moments.
Examples you can visualize
- Castling out of check is illegal:
In this position, White is in check from the bishop on b4. Castling kingside (O-O) would be illegal because you cannot castle out of check; you must first get out of check by moving the king or blocking/capturing the checking piece.
Diagram:
- Illegal en passant:
Suppose Black’s last move was ...d6 (one square). White’s attempt 1. exd6 e.p. would be illegal, because en passant is only available immediately after a two-square pawn advance that passes an adjacent enemy pawn on its fifth rank.
Diagram (illustrative squares/arrows):
- Promotion mishap:
After 1. e7-e8=Q, the pawn must be replaced by a new piece on e8 (usually a queen). Placing an inverted rook to “mean a queen” does not make it a queen under FIDE rules; it counts as a rook unless the arbiter allows correction. Failing to replace the pawn properly or attempting to promote to an illegal piece (e.g., a second king) is illegal.
Procedural notes (what to do OTB)
- If your opponent completes an illegal move and presses the clock, stop both clocks.
- Call the arbiter and state what happened. Do not move a piece yourself.
- The arbiter will restore the last legal position, apply the time penalty to your clock, and enforce touch-move if relevant.
- Play resumes from the corrected position.
Interesting facts and anecdotes
- World Blitz Championship incident (Inarkiev vs. Carlsen, 2017): Black played an illegal move delivering check while his own king was in check. Carlsen replied, the game continued, and a dispute ensued. After review, FIDE ruled that the illegal move should have been corrected, and the game was awarded to Carlsen. The case highlighted procedures for correcting illegal moves, especially in blitz.
- Upside-down rook ≠ queen: Under FIDE rules, an inverted rook used as a “makeshift queen” counts as a rook unless event rules provide a set procedure or the arbiter permits immediate correction. Always ask for proper pieces to avoid accidental illegalities.
- Chess960 pitfalls: Because starting positions vary, many “normal chess” castle attempts are illegal in Chess960. Players must know the Chess960 castling rules, which place the king and rook on their usual post-castling squares but may require different paths.
Tips to avoid illegal moves
- In check? First thought: list your three legal options—move the king, capture the checking piece, or block (if a slider).
- Before castling, verify: not in check, squares between are empty, king and rook haven’t moved, and the king does not cross a checked square.
- Track en passant availability: only on the immediate move after a two-square advance.
- Promote properly: place the new piece on the promotion square before pressing the clock.
- Use one hand per move, and never press the clock with the hand that didn’t move the piece.