Almost every move in chess follows the basic patterns in How the pieces move. Three do not. They are rare enough that many casual players have never learned them properly, and common enough that they will turn up in your games. Each one exists for a reason, and knowing the reason makes the rules easy to remember.
Castling is the only move in chess in which two pieces move at once: the king and one of the rooks. The king moves two squares towards the rook, and the rook jumps over it to land on the square right next to it, on the other side.
It does two useful jobs in a single move: it tucks the king away from the centre, where the fighting happens, and it brings a rook towards the middle, where it can join the game. That is why most games see both players castle early.
You can castle on either side. On the king’s side, known as short castling, the king ends on g1 and the rook on f1. On the queen’s side, long castling, the king ends on c1 and the rook on d1. For Black it is the same on the eighth rank.
Castling is only allowed if all of these are true:
The last rule is the one that catches people. The rook is allowed to pass through an attacked square; the king is not.
A pawn that reaches the far end of the board — the eighth rank for White, the first for Black — is promoted. It is immediately replaced by a queen, a rook, a bishop or a knight of the same colour, chosen by the player. It cannot become a king, and it cannot stay a pawn.
It does not matter what pieces are still on the board: you can have two queens, or more. Nearly every promotion is to a queen, since she is the strongest piece. Choosing anything else is called underpromotion, and it happens for a reason — usually because a knight would give check in a way the queen cannot, or because a new queen would leave the opponent with no legal moves and throw the win away as stalemate.
This is why the whole endgame so often turns into a race: a pawn that is about to promote is worth far more than the single point it counts for on paper.
On Chess Thrill you choose the piece each time a pawn promotes. If you would rather always get a queen without being asked, switch Promotion to Auto-queen in the settings.
En passant — French for “in passing” — is the rule most players learn last, and the one most often mistaken for a bug.
It exists because of the pawn’s two-square first move. Without it, a pawn could use that double step to slip past an enemy pawn that would have been able to capture it, had it moved only one square. So the rule closes the loophole: if a pawn moves two squares and lands right beside an enemy pawn, that enemy pawn may capture it as if it had moved only one.
There are three conditions, and all three must hold:
It is the only capture in chess where the capturing piece does not land on the square of the piece it takes. If you ever see a pawn vanish from beside yours in one of our games, this is almost certainly what happened.
Chess pieces: “cburnett” set by Colin M. L. Burnett, CC BY-SA 3.0.
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