Sports & Games

Chess 101: What Is Castling? Learn About the 2 Conditions That Need to Be Satisfied in Chess Before You Can Castle

Written by MasterClass

Jun 21, 2019 • 4 min read

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One of the most basic rules of chess is that you can only move a single piece at a time. This applies in every situation, except one: castling. This exceptional move is an important strategic tool that helps protect your king while also developing one of your rooks.

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What Is Castling in Chess?

Simply put, castling is a special rule that allows your king to move two spaces to its right or left, while the rook on that side moves to the opposite side of the king. FIDE, the international organization governing the rules of chess, defines castling this way:

“This is a move of the king and either rook of the same color along the player’s first rank, counting as a single move of the king and executed as follows: the king is transferred from its original square two squares towards the rook on its original square, then that rook is transferred to the square the king has just crossed.”

How Do You Castle?

Learn how to perform the castle move by considering the following example. First, disregard the other pieces on the board, and only focus on the white king and the rook on h1.

Diagram of chess move in red and pink hues

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In this scenario, neither the king nor the rook has moved, satisfying the first condition. Furthermore, there are no pieces blocking the king and the rook. The king is not in check, and we can confirm that the king won’t pass through a square that’s being attacked during the course of the move. Therefore, white may castle.

  • First, the king will move two spaces toward the rook, ending on g1.
  • Next, the rook on h1 will hop over the king to f1. A good rule general rule to remember for castling is that the king will always end on the same color square that it started from. (Put another way, the white king will always castle onto a black square, while the black king will always castle onto a white square.)
  • Because the king is castling toward the rook on its side, this is called kingside castling. (Had you instead castled toward the other rook, it would have been queenside castling.) In standard chess notation, kingside castling is notated O-O (or 0-0), while queenside castling is notated O-O-O (or 0-0-0), indicating the number of spaces the rook jumps.

What 2 Conditions Need to Be Satisfied in Chess Before You Can Castle?

There are a number of conditions that need to be satisfied before you can castle.

  1. You cannot castle if the king has already moved, or if the rook in question has moved.
  2. Nor can you castle while in check. However, you can castle with a rook that is under attack at the time, and the rook can pass through an attacked square when castling while the king cannot. (Amusingly, this was once called into question at the highest level, when a top Grandmaster, Viktor Korchnoi, went to confirm with the arbiter that he could castle with his rook under attack during a game with Karpov in 1974.)

What Are the Origins of Castling?

As with other special moves like the double-step for pawns and en passant, the origins of castling date to the late medieval period, when the modern rules of chess were being finalized. Also like those other innovations, one of the main drivers of the change was to speed up the game. Developing material is one of the major themes of the opening section of a chess game, and castling allows you to quickly get a rook toward the center of the board, which can considerably hasten their development. This is why castling figures prominently in a number of famous chess openings.

But there’s another reason for the development of this special move: it gives you a way to quickly get the king to safety. This need came about because of some other developments around the same time, namely, the development of the modern queen and bishop.

During this period, it was considered safer to keep the king near the center of the board. Once the diagonal lanes became vectors of attack however, the king became vulnerable to attacks from the flanks as well as the center.

Castling gave players a quick way to get their king to the edges of the board, protecting it from early assaults.

When Is It A Good Idea to Castle?

Castling can be a very powerful move because it’s essentially two moves at once. It can be a great way to get your king to safety while developing a powerful attacking piece in your rook. That said, knowing when to castle is critical. Here are a few tactical considerations to keep in mind:

  1. Where is your king most useful? In many situations (possibly even the majority of them), it’s better to have your king safely in the corner, where they’re less vulnerable to diagonal attacks. That makes an early castle a potentially appealing move. That said, there may be situations where a number of bishops or even queens exit the game early. In these endgame-like situations, it may be better to have the king near the center, where it may reveal itself to be a powerful attacking piece.
  2. Can you get your rooks chatting? Rooks that are connected (also called “communicating” or “chatting”) have an open rank between them. This frees them to patrol the rank, supporting other pieces freely while protecting one another.
  3. Can you disrupt your opponent’s attack? Sometimes it’s best to wait until your opponent has committed to an attack before you castle. Timed right, this can defuse your opponent’s attack while setting your own pieces up for a counterattack. Remember, while you can’t castle out of or through check, your rook can castle out of or through an attacked square.

Become a better chess player with tips and tricks from Garry Kasparov’s MasterClass.