Giving your king a safer home

Your king is the most important piece on the chessboard. Lose your queen, and you can still win. Lose your king to checkmate, and the game is over.

The trouble is that your king starts in the middle of the back rank. Once the centre opens up, that can become a rather dangerous place to live.

Fortunately, chess gives you a special move that helps protect your king and brings a rook closer to the action. It's called castling.

How to castle

Castling is officially a king move. It is the only move in chess in which two of your pieces move during the same turn, but the whole thing still counts as one move.

On a physical chessboard, the order matters:

1. Move your king first, exactly two squares towards the rook you are castling with.
2. Move that rook to the other side of your king, placing it on the square the king crossed. The rook ends up immediately next to the king.

Think: king first, two squares; rook across to the other side.

Kingside castling (O-O)

Kingside castling uses the rook on the h-file. For White, the king goes from e1 to g1; the rook goes from h1 to f1. For Black, the corresponding squares are e8 to g8 and h8 to f8.

It is sometimes called short castling. Watch the king move two squares and the rook land next to it:

Queenside castling (O-O-O)

Queenside castling uses the rook on the a-file. For White, the king goes from e1 to c1; the rook goes from a1 to d1. For Black, it is e8 to c8 and a8 to d8.

It is sometimes called long castling, because the rook travels further than it does on the kingside. The king still moves just two squares.

Black castles in exactly the same way. Here is kingside castling from Black's point of view:

When can you castle?

Castling is helpful, but you cannot do it whenever you fancy. All of these conditions must be met:

1. The king has never moved. Even if it returns to its original square, the right to castle is gone for the rest of the game.
2. The rook you are using has never moved. Moving one rook only affects castling on that side.
3. All squares between the king and that rook are empty. You usually need to develop some pieces first.
4. Your king is not currently in check. You cannot castle out of check.
5. Your king does not cross or finish on an attacked square. You cannot castle through or into check.

The restrictions on attacked squares concern the king, not the rook. A rook may be attacked, and the rook may pass through an attacked square, without that alone making castling illegal.

Castling over the board

Online, the app normally moves both pieces for you when you castle. Over a real chessboard, move the king first, then the rook, using the same hand for both pieces.

In an over-the-board game using touch-move rules, touching or moving the rook first can mean you must make a rook move instead of castling. Moving the king first makes your intention clear and follows the correct procedure.

Why castle?

Castling does two useful jobs at once.

It often gives your king more shelter. The centre can open up quickly as pawns move and pieces are exchanged. Castling usually tucks your king away behind friendly pawns, away from the busiest lines.

It helps activate a rook. Your rooks begin in the corners. Castling brings one of them closer to the centre, where it may later support an open file or work with the other rook.

Those are good reasons to castle in many openings, but don't treat it as an automatic instruction.

Should you always castle?

Not necessarily. Before choosing a side, look at where your opponent's pieces are aiming. A king hidden behind pawns isn't very safe if those pawns are already being attacked.

Ask yourself: Which side looks safer? And will the pawn shield in front of my king stay reasonably intact?

Castling Challenge

Can you tell when castling is legal? In each position, decide whether the named player can castle on the named side right now. Some positions look almost identical, so check the king, rook, empty squares and attacked squares carefully.

What to remember

• Castling is one king move involving the king and one rook.
• King first: move it two squares towards the rook, then move the rook to the other side of the king.
• O-O means kingside castling; O-O-O means queenside castling.
• The king and chosen rook must never have moved, and the squares between them must be empty.
• Your king cannot castle out of, through or into check.
• Castling can improve your king's safety and bring a rook into play, but consider which side is safest.

Knowledge check

Have you got the movement and the restrictions clear? Test yourself before moving on.