Castling

A simultaneous move of the king and rook that each side may make only once in a game. It works like this: the king moves two squares toward the rook, then the rook jumps over the king to land on the square right next to it.

Castling toward the queenside is called long castling (notation O-O-O); toward the kingside, short castling (O-O).

Rules

Castling isn't possible if:

1. the king or rook has already moved in the game;
2. the king is in check;
3. castling would put the king in check;
4. the square the king passes over is attacked by an enemy piece;
5. there are other pieces between the king and the rook it's castling toward.

Role

Castling plays a big role in both strategy and tactics. Taking the king out of the center makes it safer, while bringing the rook toward the center helps mobilize your forces. One side will often sacrifice material just to stop the other from castling, then attack the king stuck in the center.

History

Castling was first introduced in Italy in the late 15th century, and the term itself comes from the Italian name for the rook, "Rocco." But the rules we use today didn't appear right away. At first, free castling was common — the king and rook could end up in several different positions, whichever the player chose (for example, Kf1—Re1, Kh1—Rg1, and so on for short castling, Kb1—Rc1 and so on for long). By the mid-19th century, free castling had been replaced everywhere by the restricted version, except in Italy, where it held on almost to the end of the century.

In composition

In composition, castling is allowed on any move of the solution unless you can prove the king or the rook — whichever side White or Black wants to castle toward — has already moved in the game. Castling is one of the main themes in analytical problems.