Illegal moves in chess: what they are and what happens if you make one

Has your opponent ever told you “that move doesn’t count”? Let’s look at exactly what an illegal move is, which ones happen most often, and what happens in a tournament if you make one.

What is an illegal move in chess?

The idea is simple: an illegal move is any move that breaks the rules of chess.

That covers three broad categories:

1. Moving a piece in a way that doesn’t match how the pieces move.
2. Making a move that leaves your own king in check, or fails to get out of check if you were already in it.
3. Executing a special move incorrectly, such as castling: for example, castling through a square that’s under attack.

If you make one of these mistakes, the move isn’t valid. Full stop.

Practice: get out of check legally

One of the most common illegal moves among beginners is making any move without addressing the check. If your king is threatened, you can’t just play as if nothing happened. You first have to get out of check in one of these three ways:

1. Move the king to a safe square.
2. Capture the attacking piece.
3. Block the line of attack with another piece, if the position allows it.

What would you play in this position?

In this position, only Kb1 works. If you ignore the check or move the king to another attacked square, the move is illegal. This principle applies in every game, whether friendly or a tournament.

List of the most common illegal moves

Here are the concrete cases I’ve seen come up most often, especially among beginners:

1. Castling through an attacked square. Castling has clear conditions: the king can’t pass through or land on a square that’s in check. If you do, the move is illegal.
2. Leaving your own king in check. Any move that leaves your king under attack is illegal, even if the piece you moved was a different one.
3. Not getting out of check. If you’re in check and you play a move that doesn’t resolve the threat, that doesn’t count either.
4. Moving a piece outside its pattern. A bishop moving in a straight line, a knight moving in a straight line… illegal in every case.
5. Promoting without declaring a piece. When a pawn reaches the eighth rank, you must state which piece it becomes. Failing to do so is an incomplete, illegal move.

What to do if you make an illegal move

Most of the time an illegal move happens by accident. So what do you do?

If you haven’t pressed the clock, you can take it back and make a different legal move with the same piece. The touch-move rule applies here: if you touched the piece, you must move it somewhere legal.

That said, if you say “j’adoube” (I adjust) before touching it, you’re signaling that you’re only straightening the piece on its square without intending to move it. That way it doesn’t force you to play it.

If you’ve already pressed the clock, the move stands. That’s when the arbiter gets involved.

Consequences in tournaments under the FIDE rules

The International Chess Federation has this tightly regulated. The penalties depend on the chess clock and the time control of the game:

• First illegal move detected: the opponent’s clock gets 2 extra minutes in rapid or classical games; 1 minute in blitz.
• Second illegal move in the same game: immediate loss of the game.

Also remember that if your opponent makes an illegal move, you must claim it before making your own move. If you play on, the illegal move is accepted and you can no longer claim it.

The most famous illegal moves

Do you think grandmasters are immune to mistakes like this? Not at all. Several cases have even been caught on camera.

One of the best known happened when Grandmaster Veselin Topalov promoted a pawn on the eighth rank without declaring a piece, in a game against Grandmaster Garry Kasparov. Commentators pointed out that, a few years earlier, that would have meant an immediate loss. The rules had since been updated.

Once you’ve got all this down, you’ll know how to move with confidence in a game: you’ll recognize when a move is illegal, you’ll claim it when it’s your right, and you’ll avoid making these mistakes yourself.