Quick answer
Chess has three special moves. Castling moves the king two squares toward a rook, and the rook jumps to the square on the king's other side. En passant lets a pawn capture an enemy pawn that has just moved two squares past it. Promotion turns a pawn that reaches the last row into a queen, rook, bishop or knight.
Castling
Castling is the only move where two pieces move at once. The king moves two squares toward a rook, and the rook jumps over to the other side of the king. It keeps your king safe and brings the rook into play.
You can castle only if:
- neither the king nor that rook has moved yet;
- all squares between them are empty;
- the king is not in check, doesn't pass through an attacked square, and doesn't land in check.
Short castling (kingside) is written O-O; long castling (queenside) is O-O-O.
En passant
En passant (French for "in passing") is a special pawn capture. If an enemy pawn moves two squares forward and lands right beside your pawn, you can capture it as if it had moved only one square.
You must do it immediately, on the very next move, or the chance is gone.
On this board, Black has just played the pawn from d7 to d5. Your e5 pawn can take it by moving to d6.
Promotion
When a pawn reaches the last rank, it promotes: it turns into a queen, rook, bishop or knight of your choice. Almost everyone chooses a queen, but a knight or rook is sometimes better (see underpromotion).
You can have more than one queen this way.
Frequently asked questions
Can you castle if the rook is attacked?
Yes. Only the king's squares matter: the king can't be in check, pass through an attacked square or land on one. The rook may be attacked.
Is en passant mandatory?
No. It's optional, like any capture, unless it's the only legal move. But if you don't do it right away, you lose the right to do it.
Can a pawn promote to a king?
No. A pawn can become a queen, rook, bishop or knight, but never a king.