Castling, En Passant and Promotion: the three special moves

Three moves break the normal rules of the board. Castling moves two pieces at once. En passant captures a pawn on a square it is not standing on. Promotion swaps a pawn for a new piece at the far end of the board. These are the rules beginners get wrong most often, and all three show up in real games constantly.

Castling: the only move with two pieces

When you castle, the king travels two squares toward one of your rooks and that rook jumps over him, landing on the square next door. It is the only move where two of your own pieces move together, and the only one where the king travels more than one square. It exists for a practical reason: it pulls the king out of the center, where lines open first, and brings a rook toward a useful file.

1. Your king has never left its starting square, and neither has the rook on that side. Moving away and coming back burns the right.
2. Every square between the king and that rook is empty.
3. Your king is not in check right now.
4. Your king does not pass through any square your opponent attacks.
5. Your king does not land on an attacked square: castling into check does not exist.

Kingside and queenside castling

Castling with the h-rook is kingside or short castling, written O-O (the letter O, never a zero): the king goes e1 to g1 and the rook h1 to f1, and for Black e8 to g8 and h8 to f8. Castling with the a-rook is queenside or long castling, O-O-O: king from e1 to c1, rook from a1 to d1. The long one needs three empty squares instead of two, and it leaves the a-pawn more exposed.

In practice, castle early, usually between moves 5 and 10: a king sitting in the center while the files open is the classic way to lose in twenty moves. And the right is lost rook by rook, not in one block: if your h1 rook has moved, queenside castling still works. Only the king erases both.

En passant: the capture in passing

En passant patches a hole that opened when pawns could first advance two squares in one move. Without it, that jump would let a pawn slip past a square guarded by an enemy pawn and escape. En passant hands back the skipped capture: you take as if the other pawn had only moved one square.

The part that trips people up is geometric: the capturing pawn lands on an empty square, and the pawn it removes was sitting beside that square. It is the only capture in chess where the piece leaving the board was never on the destination square.

• The capturing piece is always a pawn standing on its own fifth rank: rank 5 for White, rank 4 for Black.
• The captured pawn has just advanced two squares in one move, straight from its starting square, landing on the file beside yours.
• The capture is immediate, on your very next move. There is no en passant against a pawn that walked up one square at a time.

Promotion: when a pawn becomes a queen

A pawn reaching the far rank (rank 8 for White, rank 1 for Black) has nowhere to go, so it is exchanged on the spot for another piece of its own color: queen, rook, bishop or knight. A king is not allowed, and neither is staying a pawn. Two or three queens at the same time is perfectly legal. The swap is part of the move: if the new piece gives check, the check counts right there.

Notation is short. A promotion is written e8=Q, using the standard piece letters K, Q, R, B and N. If the pawn promotes by capturing, the starting file comes first, as in exd8=Q+. Older books sometimes put the new piece in parentheses, but the equals sign is today's standard.

Underpromotion: when a knight is the right answer

The queen is almost always the best piece, because she does everything a rook and a bishop can do. The real exception is the knight, the one piece capable of a move no queen can make. Promoting to a knight pays off when it gives check and attacks something valuable in the same move.

A queen on d8 would give no check at all: Black keeps the queen and material stays level. The knight wins a piece outright. There is also underpromotion to a rook, for endgames where a new queen would leave the enemy king with no legal move and not in check: that is stalemate, and stalemate is a draw. A rook covers less ground and the king still has a move.

The most common mistakes

• Assuming one rook move kills both castles. It only kills that side.
• Trying en passant one move too late. The right has expired and the board refuses it.
• Aiming at the wrong square: the pawn goes to the square your opponent crossed, not where the pawn stopped.
• Promoting on autopilot and stalemating the enemy king. Check that the other side still has a legal move.