In chess notation, the letter M is used to indicate a special move called a castling move. Castling is a unique move in chess that allows a player to move their king and one of their rooks simultaneously. It is the only time in chess when a player can move two of their own pieces in one move.
The Castling Move
Specifically, when a player castles, they move their king two squares towards one of their rooks, and then move that rook to the square on the other side of the king. This allows the player to get their king to safety and also activate one of their powerful rooks.
There are a few requirements for castling:
- It must be the king’s and rook’s first move of the game
- There cannot be any pieces between the king and rook
- The king cannot be in check
- The king cannot pass through check during the castle
If these conditions are met, then a player may castle kingside or queenside. Kingside castling refers to castling towards the king’s side of the board (towards the h-file) while queenside castling refers to castling towards the queen’s side (towards the a-file).
Castling Notation
When a player castles in chess notation, it is indicated by the letter O or O-O for kingside castling, and O-O-O for queenside castling. For example:
- O-O = Kingside castling
- O-O-O = Queenside castling
The letter O stands for castling in French (“roque”). So O-O shows the king moving two spots towards the rook on the king’s side, while O-O-O shows the king moving two spots towards the rook on the queen’s side.
Examples of Castling
Here are some examples to illustrate castling notation:
- 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O = White kingside castles on move 5
- 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Be3 e5 7. Nb3 Be6 8. O-O-O = Black queenside castles on move 8
In the first example, White castles kingside in algebraic notation. In the second example, Black castles queenside. The O-O and O-O-O makes it clear that these were castling moves.
Purpose of Castling
Castling serves two main purposes in chess:
- It protects the king by moving it towards the corner of the board away from the center.
- It activates the rook by moving it to a more central position.
A safe king and active rooks are crucial for controlling the game, so castling plays an important role in almost every chess opening. Good players know how vital it is to castle early on.
Castling in Chess Openings
Since castling is so useful, most openings start with the moves:
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O
This allows White to castle kingside right away on move 5. Black often responds by castling on move 7 or move 8.
Certain openings favor queenside castling instead, like the Queen’s Gambit:
1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 Be7 5. e3 O-O 6. Nf3 Nbd7 7. Rc1 O-O-O
Here Black decides to castle queenside to keep their king safe on the queenside where White’s pawn attack on the kingside cannot reach it.
Deferred Castling
While castling early is commonplace, sometimes players will intentionally defer castling in order to castle either kingside or queenside depending on where the opponent attacks. Delaying castling allows for more flexibility.
For example, in the Ruy Lopez:
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 d6 8. c3 O-O 9. h3 Nb8
Here Black delays castling kingside until move 9, allowing them to see what White is doing before deciding where to castle.
Kingside vs Queenside Castling
In general, kingside castling is safer because the king is not as exposed on that side of the board. However, queenside castling has the advantage of allowing the queen and rook to switch places and connect.
Kingside castling is more common, but openings like the Sicilian Defense with an early c5 by Black encourage queenside castling from White in response.
Castling to Attack
While castling is primarily a defensive move, it can also facilitate an attack by bringing a rook into the game quickly.
In the Fried Liver Attack against the Two Knights Defense, White castles queenside and then sacrifices a knight on f7 to launch a mating attack:
1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4. Ng5 d5 5. exd5 Nxd5 6. Nxf7 Kxf7 7. Qf3+ Ke6 8. Nc3 Nb4 9. O-O-O
Here the queenside castling helps White bring their a1 rook into the attack.
Fianchetto Castling
Some openings feature fianchetto castling where the king castles to the side with a fianchettoed bishop. This allows the bishop and rook to control key diagonals and files.
For example, in the King’s Indian Defense:
1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. Nf3 O-O 6. Be2 e5 7. O-O Nbd7
Black fianchettoes their bishop and then castles kingside to support the bishop and control the g-file with the rook.
Castling into Attack
While risky, sometimes a player will intentionally castle into an attack as a way to challenge their opponent. If the attack is not completely sound, castling towards it can create counterplay.
For example, in the Fried Liver Attack, Black sometimes castles kingside to challenge White’s initiative:
1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4. Ng5 d5 5. exd5 Nxd5 6. Nxf7 Kxf7 7. Qf3+ Ke6 8. Nc3 Nb4 9. O-O
By castling into the attack, Black risks getting mated but also challenges White to find the most accurate continuation.
When Not to Castle
While most games feature castling from both sides, occasionally a player will intentionally not castle for strategic reasons. Some examples:
- To perform a kingside pawn storm before castling kingside
- Leaving the king in the center to facilitate counterattacks in the middlegame
- Avoiding getting trapped after castling if the opponent is preparing an attack against the castled position
- In hyper-aggressive openings where neither side castles in order to attack as quickly as possible
So while castling is almost always a good idea, strong players understand when delaying or avoiding castling altogether makes sense.
Special Chess Rules About Castling
There are a few special rules about castling that are important to keep in mind:
- You cannot castle out of, through, or into check
- Castling counts as a king move – you can only do it once per game with each king
- The king and rook must be on their original squares to castle
- You cannot castle with a promoted rook
As long as you follow these rules, you will be castling correctly in chess!
Castling in Chess Puzzles
Castling shows up often in chess puzzles and compositions. Some examples of creative castling puzzles:
- Castling to block a check or create an escape square
- Castling to free a pinned piece and solve a problem
- Taking advantage of a castling path that crosses an attacked square multiple times
- Castling early in a puzzle to activate a rook for the key move
Solvers have to consider all crafty castling possibilities when working through chess puzzles.
Famous Chess Games Featuring Castling
Here are some famous chess games where creative and unusual castling played a key role:
- Anderssen – Dufresne, 1852 – Anderssen castles queenside into an attack to set up a mating net.
- Byrne – Fischer, 1956 – Fischer castles queenside to allow a sacrifice leading to a king hunt.
- Karpov – Kasparov, 1985 – Karpov castled kingside to allow a knight sacrifice and attack against Kasparov’s king.
- Shirov – Anand, 1998 – Shirov castled long on move 4 to launch an ultra-aggressive king hunt against Anand’s king.
The importance of castling is evident in many classic games, highlighting the importance of understanding this unique chess move.
Common Castling Mistakes
There are some notable mistakes players often make when castling:
- Castling into check – an illegal move that results in having to move the king somewhere else instead
- Castling through check – also illegal, as the king cannot cross attacked squares
- Forgetting that the king or rook has already moved – castling requires the original king and rook position
- Attempting to castle with a promoted piece – only original rooks allow castling
- Castling too late – delayed castling can leave the king dangerously exposed
Being aware of these common errors can help you avoid making similar mistakes in your own games.
Importance of Understanding Castling
While a seemingly simple maneuver, castling is immensely important in chess strategy and requires some nuance to fully understand.
Mastering castling rules, notation, patterns, and strategic concepts will surely improve your overall chess game. Castling may be the most important thing every chess player needs to learn
Conclusion
In summary, the letter M in chess notation refers to the special castling move involving the king and rook. Correctly executing castling allows you to safely activate your king and rook early in the game. While there are some rules around when you can and cannot castle, it remains a vital strategic idea in the majority of chess openings. Understanding castling notation, patterns, and concepts is key to elevating your overall chess skill.