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Castling is a move in chess. It consists of moving the king two squares toward a rook on the same rank and then moving the rook to the square that the king passed over.[2] Castling is permitted only if neither the king nor the rook has previously moved; the squares between the king and the rook are vacant; and the king does not leave, cross over, or finish on a square attacked by an enemy piece. Castling is the only move in chess in which two pieces are moved at once.[3]
Initial positions of kings and rooks. Kings may castle to the indicated squares.
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White has castled kingside; Black has castled queenside.
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Castling with the king's rook is called kingside castling, and castling with the queen's rook is called queenside castling. In both algebraic and descriptive notations, castling kingside is written as 0-0 and castling queenside as 0-0-0.
Castling originates from the king's leap, a two-square king move added to European chess between the 14th and 15th centuries, and took on its present form in the 17th century. Local variations in castling rules were common, however, persisting in Italy until the late 19th century. Castling does not exist in Asian games of the chess family, such as shogi, xiangqi, and janggi, but it commonly appears in variants of Western chess.
Description
During castling, the king is shifted two squares toward a rook of the same color on the same rank, and the rook is transferred to the square crossed by the king. There are two forms of castling:[4]
- Castling kingside (short castling) consists of moving the king to g1 and the rook to f1 for White, or moving the king to g8 and the rook to f8 for Black.
- Castling queenside (long castling) consists of moving the king to c1 and the rook to d1 for White, or moving the king to c8 and the rook to d8 for Black.
Requirements
White cannot castle queenside because the knight is in the way. White can castle kingside.
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Black cannot castle on either side because the black king is in check by the white bishop.
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White cannot castle queenside because Black's queen controls c1. White can castle kingside even though the h1-rook is under attack.
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Black cannot castle kingside because the white queen attacks f8. Black can castle queenside despite the white queen's attack on b8.
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Castling is permitted provided all of the following conditions are met:[5]
- Neither the king nor the rook has previously moved.
- There are no pieces between the king and the rook.
- The king is not currently in check.
- The king does not pass through or finish on a square that is attacked by an enemy piece.
Conditions 3 and 4 can be summarized by the mnemonic: A player may not castle out of, through, or into check.
Castling rules often cause confusion, even occasionally among high-level players.[6] Alexander Beliavsky and Viktor Korchnoi both had to consult the arbiter during tournaments on whether castling was legal when the rook was on or passed over an attacked square, Yuri Averbakh once mistakenly thought that Black queenside castling was illegal when b8 was attacked, and Nigel Short once attempted to castle queenside as Black when d8 was under attack (this was not allowed). Illegal castling has also occasionally occurred in serious games between top players (including Gata Kamsky, Viktor Korchnoi, and Richard Réti) when they forgot that the king or rook had previously moved and returned to its home square, and has not always been noticed by the opponent. Yasser Seirawan once even accidentally castled queenside as White with his queen's rook on b1 (which was not allowed), and Alexander Alekhine once "castled his queen" (moving his queen from d1 to b1 and his rook from a1 to c1, which was also not allowed).[7] To clarify:
- The rook can be under attack.
- The rook can pass through an attacked square. (White can castle queenside even if Black is attacking b1; Black can castle queenside even if White is attacking b8.)
- The king can have been in check earlier in the game.
Tournament rules
Under FIDE rules and USCF rules, and enforced in most tournaments, castling is considered a king move, so the king must be touched first; if the rook is touched first, a rook move must be played instead. As usual, the player may choose another legal destination square for the king until releasing it. When the two-square king move is completed, however, the player is committed to castling if it is legal, and the rook must be moved accordingly. The entire move must be completed with one hand. A player who attempts to castle illegally must return the king and rook to their original squares and then make a legal king move if possible (which may include castling on the other side). If there is no legal king move, the touch-move rule does not apply to the rook.[8][9]
These tournament rules are not commonly enforced in informal play nor commonly known by casual players.[10]
Castling rights
An unmoved king has castling rights with an unmoved rook of the same color on the same rank, even if castling is not legal in that particular position. In the context of threefold and fivefold repetition, two positions with different castling rights are considered to be different positions.
Example of castling rights
Karpov vs. Miles, 1986
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Position after 22.Nb5
In a 1986 game between Anatoly Karpov and Tony Miles,[11] play continued from the diagrammed position as follows:
- 22... Ra4 23. Nc3 Ra8 24. Nb5 Ra4 25. Nc3 Ra8 26. Nb5
With his 26th move, Karpov attempted to claim a draw by threefold repetition, thinking that the positions after his 22nd, 24th, and 26th moves were the same. It was pointed out to him, however, that the position after his 22nd move had different castling rights than the positions after his 24th and 26th moves, rendering his claim illegal. As a result, Karpov was penalized five minutes on his clock. After thinking for about ten minutes, Miles decided to agree to a draw anyway (even an incorrect claim of threefold repetition is also a draw offer).[7]
Both algebraic notation and descriptive notation indicate kingside castling as 0-0 and queenside castling as 0-0-0 (using the digit zero). Portable Game Notation and some publications use O-O for kingside castling and O-O-O for queenside castling (using the letter O) instead. ICCF numeric notation indicates castling based on the starting and ending squares of the king; thus, castling kingside is written as 5171 for White and 5878 for Black, and castling queenside is written as 5131 for White and 5838 for Black.
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Castling under medieval rules. Symbol color corresponds to king color. Circles represent English, Spanish, French, and Lombardy movement; crosses represent Lombardy movement only.
Castling has its roots in the king's leap. There were two forms of the leap: the king would move once like a knight, or the king would move two squares on its first move. The knight move might be used early in the game to get the king to safety or later in the game to escape a threat. This second form was played in Europe as early as the 13th century. In North Africa, the king was transferred to a safe square by a two-move procedure: the king moved to the player's second rank, and the rook and king moved to each other's original squares.[12]
Various forms of castling were developed due to the spread of rulesets during the 15th and 16th centuries which increased the power of the queen and bishop, allowing these pieces to attack from a distance and from both sides of the board, thus increasing the importance of king safety.[13]
The rule of castling has varied by location and time. In medieval England, Spain, and France, the white king was allowed to jump to c1, c2, d3, e3, f3, or g1[14] if no capture was made and the king was not in check and did not move over check; the black king might move analogously. In Lombardy, the white king might also jump to a2, b1, or h1, with corresponding squares applying to the black king. Later, in Germany and Italy, the rule was changed such that the king move was accompanied by a pawn move.
In the Göttingen manuscript (c. 1500) and a game published by Luis Ramírez de Lucena in 1498, castling consisted of moving the rook and then moving the king on separate moves.
The current version of castling was established in France in 1620 and in England in 1640.[15] It served to combine the rook's move and the king's jumping move into a single move.[16]
In Rome, from the early 17th century until the late 19th century, the rook might be placed on any square up to and including the king's square, and the king might be moved to any square on the other side of the rook. This was called free castling.
In the 1811 edition of his chess treatise, Johann Allgaier introduced the 0-0 notation. He differentiated between 0-0r (right) and 0-0l (left). The 0-0-0 notation for queenside castling was introduced in 1837 by Aaron Alexandre.[17] The practice was adopted in the first edition (1843) of the influential Handbuch des Schachspiels and soon became standard. In English descriptive notation, the word "Castles" was originally spelled out, adding "K's R" or "Q's R" if disambiguation was needed; eventually, the 0-0 and 0-0-0 notation was borrowed from the algebraic system.
Strategy
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Position after 10.0-0-0: Opposite castling in the Yugoslav Attack
Castling is generally an important goal in the opening: it moves the king to safety away from the center files of the board, and it moves the rook to a more active position (the f-file if castling kingside; the d-file if castling queenside).
The choice regarding to which side one castles often hinges on an assessment of the trade-off between king safety and activity of the rook. Kingside castling is generally slightly safer because the king ends up closer to the edge of the board and can usually defend all of the pawns on the castled side. In queenside castling, the king is placed closer to the center and does not defend the pawn on the a-file; for these reasons, the king is often subsequently moved to the b-file. In addition, queenside castling is initially obstructed by more pieces than kingside castling, thus taking longer to set up than kingside castling. On the other hand, queenside castling places the rook more efficiently on the central d-file, where it is often immediately active; meanwhile, with kingside castling, a tempo may be required to move the rook to a more effective square.
Players may forgo castling for various reasons. In positions where the opponent cannot organize an attack against a centralized king, castling may be unnecessary or even detrimental. In addition, in certain situations, a rook can be more active near the edges of the board than in the center; for example, if it is able to fight for control of an open or semi-open file.
Kingside castling occurs more frequently than queenside castling. It is common for both players to castle kingside, somewhat uncommon for one player to castle kingside and the other queenside, and somewhat rare for both players to castle queenside. If one player castles kingside and the other queenside, it is called opposite castling or opposite-side castling. Castling on opposite sides usually results in a fierce fight, as each player's pawns are free to advance to attack the opponent's castled position without exposing the player's own castled king. Opposite castling is a common feature of many openings, such as the Yugoslav Attack.
Tactics involving castling
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Black to play
Tactical patterns involving castling are rare. One pattern involves castling queenside to deliver a double attack: the king attacks a rook (on b2 for White or b7 for Black), while the rook attacks a second enemy piece (usually the king). In the example shown, from the game Mattison–Millers, Königsberg 1926,[18] Black played 13...Rxb2?? and resigned after 14.0-0-0+, which wins the rook.
Chess historian Edward Winter has proposed the name "Thornton castling trap" for this pattern, in reference to the earliest known example, Thornton–Boultbee, published in the Brooklyn Chess Chronicle in 1884. Other chess writers such as Gary Lane have since adopted this term.[19]
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White to play
Another example of tactical castling is illustrated in the diagrammed position from the correspondence game Gurvich–Pampin, 1976. After 1.Qxd8+ Kxd8 2.0-0-0+ Ke7 3.Nxb5, White has won a rook by castling with check and simultaneously unpinning the knight.[20]
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Black to play
Such a double attack can also be made by castling kingside, although this is much rarer. In this position from the blindfold game Karjakin–Carlsen, 2007, the move 19...0-0 threatens to win the rook on h7, as well as 20...Bxg5, when White cannot recapture due to the threat of back rank mate. Black will thus win the g5-knight next move; 20.Rh6 Bxg5 21.Rxg6+ Kh7 22.Rxg5 would not work, as it would be met by 22...Rf1#.[21]
Korchnoi vs. Karpov
Viktor Korchnoi, in his 1974 Candidates final match with Anatoly Karpov, asked the arbiter if castling was legal when the castling rook was under attack.[22] The arbiter answered in the affirmative, Korchnoi executed the move, and Karpov resigned shortly after.[23]
Heidenfeld vs. Kerins
Castling occurred three times in the game Wolfgang Heidenfeld–Nick Kerins, Dublin 1973. The third instance of castling, the second one by White, was illegal, as the white king had already moved. The game is as follows:[24]
1.e4 e6 2.d4 d5 3.Be3 Nf6 4.e5 Nfd7 5.f4 c5 6.c3 Nc6 7.Nf3 Qb6 8.Qd2 c4 9.Be2 Na5 10.0-0 f5 11.Ng5 Be7 12.g4 Bxg5 13.fxg5 Nf8 14.gxf5 exf5 15.Bf3 Be6 16.Qg2 0-0-0 17.Na3 Ng6 18.Qd2 f4 19.Bf2 Bh3 20.Rfb1 Bf5 21.Nc2 h6 22.gxh6 Rxh6 23.Nb4 Qe6 24.Qe2 Ne7 25.b3 Qg6+ 26.Kf1 Bxb1 27.bxc4 dxc4 28.Qb2 Bd3+ 29.Ke1 Be4 30.Qe2 Bxf3 31.Qxf3 Rxh2 32.d5 Qf5 33.0-0-0 Rh3 34.Qe2 Rxc3+ 35.Kb2 Rh3 36.d6 Nec6 37.Nxc6 Nxc6 38.e6 Qe5+ 39.Qxe5 Nxe5 40.d7+ Nxd7 0–1
Averbakh vs. Purdy
Averbakh vs. Purdy, 1960
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Black to move castled queenside, with the rook traversing the attacked b8-square.
In the game Yuri Averbakh–Cecil Purdy, Adelaide 1960,[25] when Purdy castled queenside, Averbakh queried the move, pointing out that the rook had passed over an attacked square. Purdy indicated e8 and c8 and said, "The king", in an attempt to explain that this was forbidden only for the king. Averbakh replied, "Only the king? Not the rook?" Averbakh's colleague Vladimir Bagirov then explained the castling rules to him in Russian, and the game continued.[26][27][28]
Edward Lasker vs. Thomas
Ed. Lasker vs. Thomas, 1912
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Position after 17...Kg1
In the game Edward Lasker–Sir George Thomas (London 1912),[29] White could have checkmated with 18.0-0-0#, but he instead played 18.Kd2#.[30] (See Edward Lasker's notable games.)
Prins vs. Day
Prins vs. Day, 1968
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White to play
The diagram shows the final position of the game Lodewijk Prins–Lawrence Day (1968), where White resigned.[31] Had the game continued, Black could have checkmated by castling:
- 29. Kf6 Qf5+ 30. Kg7 Qg6+ 31. Kh8 0-0-0#
Feuer vs. O'Kelly
Feuer vs. O'Kelly, 1934
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The game ended 10....Rxb2 11.dxe5 dxe5?? 12.Qxd8+ Kxd8 13.0-0-0+ and O'Kelly resigned since the rook is lost.
In the 1934 Belgian Championship,[32] Otto Feuer caught Albéric O'Kelly in the Thornton castling trap. In the position in the diagram, the game continued 10...Rxb2 11.dxe5 dxe5?? 12.Qxd8+ Kxd8 13.0-0-0+, and O'Kelly resigned. Feuer's last move simultaneously gave check and attacked the rook on b2.
Fischer vs. Najdorf
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