Castling

So far, every move in chess has been one piece at a time. Meet the exception. Castling moves your king AND a rook in a single turn, and it is the move that keeps kings alive.

White is ready to castle kingside: the king slides two squares, e1 to g1, and the rook hops over him to f1.

The rule: the king moves two squares toward a rook, and that rook jumps to the square the king just crossed. Both together, one single move. Done toward the h-rook it is called castling kingside, and written O-O:

Castled. The king sits on g1 behind an unbroken pawn wall, and the rook on f1 is suddenly in the middle of the action. In notation: O-O.

You may also castle toward the a-rook: the king goes e1 to c1 and the rook lands on d1. That is castling queenside, written O-O-O. The king still travels exactly two squares; it is the rook that makes the longer trip:

Queenside castling: king e1 to c1, rook a1 to d1. Written O-O-O.

Why does this move exist? Because it does two chores in one turn: the king leaves the dangerous centre for a sheltered corner, and a sleepy corner rook wakes up near the middle files. Almost every good game includes it. One habit worth copying from the start: when you castle at a real board, move the king first, then the rook.