7 Chess Moves That Look Illegal but Are 100% Legal

Every chess player has accused an opponent of cheating at least once. Usually, the opponent was fine. The rules are just weird. Here are seven chess moves that look completely illegal and are 100% legal, plus one bonus rule and three moves that really are illegal.

Each diagram can be stepped through with the ▶ button. If you prefer to watch first, the full video is at the top of the page.

1. En passant: the pawn that captures “in passing”

En passant (French for “in passing”) is a special pawn capture. When an enemy pawn moves two squares from its starting square and lands right next to your pawn, you may capture it as if it had moved only one square.

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1.e4a62.e5d53.exd6

The rules in short:

• Only a pawn can capture en passant, and only an enemy pawn that just made a two-square move.
• Your pawn must be on its fifth rank (rank 5 for White, rank 4 for Black), right next to the enemy pawn.
• You must do it on the very next move. Wait one turn, and the chance is gone forever.
• Your pawn moves diagonally to the square the enemy pawn skipped, and the enemy pawn is removed from the board.

Why does this rule exist? When pawns gained the two-square first move, en passant made sure they could not use it to sneak past an enemy pawn without risk.

En passant: French for “in passing”, and also for “why did my pawn just disappear”. It is legal. I checked. Twice.

Short · 1:00 · NewEn Passant: The Rule Everyone Thinks Is FakeWatch the Short →

2. Castling: the king moves two squares and the rook jumps

Castling is the only move in chess where two of your pieces move at once. The king moves two squares toward a rook, and the rook jumps over the king to the square next to it.

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1.e4e52.Nf3Nc63.Bc4Nf64.O-O

Castling on the king’s side is called castling short (O-O). On the queen’s side it is castling long (O-O-O), because the rook travels three squares instead of two.

You may castle only if all of these are true:

1. Neither the king nor that rook has moved yet in this game.
2. There are no pieces between the king and the rook.
3. Your king is not in check.
4. The king does not pass through or land on a square attacked by an enemy piece.

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Short · 0:33 · 1.3K viewsCastle or Not? Only One of These Is LegalWatch the Short →

3. The rook’s loophole: castling past an attacked square

Here is the part that starts arguments. When castling queenside, the rook passes over b1 (or b8 for Black), and that square may be under attack. Only the king’s path has to be safe. The same goes for the rook itself: if only the rook is attacked, you can still castle.

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1.O-O-O

Nobody asked the rook. The rules protect the king, not his coworkers.

4. A pinned piece can still move

A piece is pinned when moving it would expose your own king to attack. Many beginners think a pinned piece is frozen. It is not. A pinned piece may move along the line of the pin, and it may even capture the piece that is pinning it.

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1.Rxe8+

What a pinned piece cannot do is step off the line, because that would leave its own king in check. A pinned knight can never move, and a bishop pinned along a file or rank cannot move either, because neither piece can travel along that line.

5. Promoting to a knight (underpromotion)

When a pawn reaches the last rank, it must be promoted, but it does not have to become a queen. You can choose a queen, rook, bishop, or knight. Choosing anything other than a queen is called underpromotion, and sometimes it is the best move on the board.

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1.e8=N+

Two common reasons to underpromote:

• A knight can give check or fork pieces in a way a queen cannot, as in the diagram.
• A new queen would sometimes cause stalemate, while a rook still wins.

The only piece a pawn can never become is a king.

6. Two queens (or nine, technically)

Promotion does not care what you already have. Promote a pawn while your first queen is still on the board, and you now have two queens. In theory you could have nine. The same goes for a third rook or a third knight.

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1.b8=Q+

In casual games, players often use an upside-down rook as the second queen. In a tournament game, an upside-down rook counts as a rook, so stop the clock and ask the arbiter for a real queen.

7. The king captures the piece giving check

If a piece gives check from right next to the king and nobody protects it, the king may simply capture it. This works with any unprotected enemy piece next to the king, not only the one giving check.

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1.Kxe2

But the king can never capture a protected piece, because that would mean moving into check.

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He is nervous, not helpless. Give the king an unprotected piece next to him and he turns into a very small, very angry forklift.

Bonus rule: stalemate

Stalemate is not a move, but it feels illegal to every player who has thrown away a winning position. If it is your turn, your king is not in check, and you have no legal moves, the game is a draw, even if your opponent has a queen and you only have a king.

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So when you are winning, always leave the enemy king a square to breathe until you are ready to deliver checkmate.

Bonus: three moves that really are illegal

To be fair, here are three moves that are illegal no matter how confident your opponent looks:

Move	Why it is illegal
Castling out of check	You cannot castle while your king is in check. Deal with the check first.
Moving a pinned piece off the pin line	It would leave your own king in check.
Ignoring a check	When your king is attacked, you must move the king, block, or capture the attacker right away.

Quick summary

Move	Legal?	The rule in one line
En passant	Yes	Capture a pawn that just jumped two squares past yours, on the very next move only.
Castling	Yes	King moves two squares, rook jumps over it. King may not be in check or cross an attacked square.
Castling past an attacked b1	Yes	Only the king’s path must be safe.
Moving a pinned piece	Yes	Only along the line of the pin.
Promoting to a knight	Yes	Queen, rook, bishop, or knight. Never a king.
Two queens	Yes	Promotion ignores what you already have.
King captures the checker	Yes	Only if the attacker is unprotected.

Ready to test yourself? Try today’s chess puzzle, or start from zero with every chess piece explained.