Chess has three moves that break the normal pattern. Each exists for a reason, and each has a condition that trips people up. Here is the full picture.
Castling
Castling moves two pieces in a single turn. The king travels two squares toward a rook, and the rook hops over him to the square he passed. Kingside it is written O-O; queenside, where the king still moves two squares but the rook travels three, it is O-O-O.
It exists because the original rules left the king in the centre with no fast way to safety, which made games long and one-sided. Castling was added so that both sides could tuck the king away and connect the rooks quickly.
The four conditions
- Neither the king nor that particular rook has moved at any point in the game. Moving and moving back does not restore the right.
- All squares between the king and rook are empty.
- The king is not currently in check.
- The king does not pass through, or land on, a square attacked by an enemy piece.
What people get wrong
The most common misconception is that the rook must also be safe. It does not. The rook may be attacked, and when castling queenside the rook may pass through an attacked square — b1 or b8 — perfectly legally. Only the king's squares matter.
The second is that you lose castling rights if the king is ever in check. You do not. You lose them only if the king actually moves.
En passant
French for 'in passing'. When a pawn uses its two-square opening jump and lands directly beside an enemy pawn, that enemy pawn may capture it as though it had only moved one square — moving diagonally to the square it skipped and removing it.
The two conditions people forget
- It is only available on the very next move. Miss it once and the right is gone for good.
- Only a pawn can do it, and only a pawn on the fifth rank (for White) or fourth rank (for Black) can be the capturer.
The rule exists because the two-square jump was a later addition. Without en passant, a pawn could use the jump to slip past a defending pawn that would otherwise have stopped it — which would have quietly broken every pawn structure in the game.
Promotion
A pawn reaching the eighth rank must immediately be replaced by a queen, rook, bishop or knight of its own colour. It is not optional and you cannot leave it as a pawn. The new piece appears on the promotion square and acts immediately, so a promoted knight can give check on the same move.
You are not limited by what has been captured. Two queens on the board at once is completely legal, as is three or nine.
Underpromotion
Choosing something other than a queen is called underpromotion, and there are exactly two situations where it makes sense. A knight, because a knight reaches squares a queen cannot, and a knight promotion with check can win the game outright. Or a rook, to avoid stalemate — if promoting to a queen would leave the enemy king with no legal move, a rook keeps the position alive.
The stalemate underpromotion
If the enemy king is on h8 and your pawn reaches g8, a queen may be stalemate while a rook is mate. Always check the king's available squares before choosing.
How the board here handles them
All three are built into the rules engine. Castling appears as a legal destination two squares away when the conditions are met, en passant shows as a capture on the empty square behind the passing pawn, and promotion opens a small dialog so you can pick the piece rather than being forced into a queen.
If a move you expect is not offered, one of the conditions above is failing. Most often it is the king passing through an attacked square when castling.