Castling

Castling is a move in the game of chess involving a player's king and either of the player's original rooks. It is the only move in chess in which a player moves two pieces in the same move, and it is the only move aside from the knight's move where a piece can be said to "jump over" another.
Castling consists of moving the king two squares towards a rook on the player's first , then moving the rook to the square that the king crossed. Castling may be done only if the king has never moved, the rook involved has never moved, the squares between the king and the rook involved are unoccupied, the king is not in check, and the king does not cross over or end on a square attacked by an enemy piece. Castling is one of the rules of chess and is technically a king move .
The notation for castling, in both the descriptive and the algebraic systems, is 0-0 with the rook and 0-0-0 with the rook; in PGN, O-O and O-O-O are used instead. Castling on the kingside is sometimes called "castling short" (the rook moves a short distance, i.e. two squares) while castling on the queenside is called "castling long" (the rook moves a long distance, i.e. three squares) .
Castling was added to European chess in the 14th or 15th century and did not develop into its present form until the 17th century. The Asian versions of chess do not have such a move.
Requirements.
Castling is permissible provided all of the following conditions hold:
Conditions 4 through 6 can be summarized with the more memorable phrase: "One may not castle out of, through, or into check."
It is a common misconception that the requirements for castling are even more stringent than the above. To clarify:
In handicap games where odds of a rook are given, the player giving odds may still castle with the absent rook, moving only the king.
Strategy.
Castling is an important goal in the opening, because it serves two valuable purposes: it often moves the king into a safer position away from the of the board, and it moves the rook to a more active position in the center of the board.
The choice as to which side to castle 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 often all the pawns on the castled side are defended by the king. In queenside castling, the king is placed closer to the center and the pawn on the a- is undefended; the king is thus often later moved to b1 to defend the a-pawn and to move the king away from the center of the board. In addition, queenside castling requires moving the queen, if not already done; therefore, it may take slightly longer to achieve than kingside castling. On the other hand, queenside castling places the rook more efficiently – on the central d-file. It is often immediately active, whereas with kingside castling a tempo may be required to move the rook to a more effective square.
It is common for both players to castle kingside, and rare for both players to castle queenside. If one player castles kingside and the other queenside, it is called "opposite" (or "opposite-side") "castling". Castling on opposite sides usually results in a fierce fight, as both players' pawns are free to advance to attack the opposing king's castled position without exposing the player's own castled king. An example is the Yugoslav Attack, in the Dragon Variation of the Sicilian Defence.
If the king is forced to move before it has the opportunity to castle, the player may still wish to maneuver the king towards the edge of the board and the corresponding rook towards the center. When a player takes three or four moves to accomplish what castling would have accomplished in one move, it is sometimes called "artificial castling", or "castling by hand".
Tactics.
Castling is thought of as a strategic concept, related to development and connecting of the rooks. However, under special circumstances castling may be a tactical idea. Thus, in the diagram on the right, after the moves 1.Qxd8+ Kxd8 2.O-O-O+ Ke7 3. Nxb5 (and White is up a rook).
Tournament rules.
Under the strict touch-move rules enforced in most tournaments, castling is considered a king move and thus the king must be touched first; if the rook is touched first, a rook move must be made instead. Under current US Chess Federation rules, however, a player who intends to castle and touches the rook first would suffer no penalty, and would be permitted to castle, provided castling is legal in the position. Still, the correct way to castle is to move the king first. As usual, the player may change his or her mind to another legal destination square for the king until it is released. 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. A player who performs a forbidden castling must return the king and the rook to their original places and then move the king, if there is another legal king move, including castling on the other side. If there is no legal king move, the touch-move rule does "not" apply to the rook .
The official rules also require that the entire move be completed using only a single hand. Neither of these rules is commonly enforced in casual play, nor commonly known by non-competitive players .
The right to castle must be the same on all three occasions for a valid draw claim under the threefold repetition rule.
Castling in chess problems.
In chess problems, castling is assumed to be allowed if it appears possible, unless it can be proved by retrograde analysis that either the king or the chosen rook had previously moved.
Castling in other games of the chess family.
Western chess variants.
Variants of Western chess have several ways to implement castling in their set of rules.
In variants played on a standard 8×8 board, castling is often the same as in standard chess. However, there exist games, such as losing chess, that do not have castling at all. A notable variant is chess960, which modified the rules of castling to support different starting positions.
Castling can also be adapted to variants with different board sizes. Some such variants, like Capablanca Chess (10×8) or Chess on a Really Big Board (16×16), preserved the castling movement of the rooks, meaning it is the king that moves further along the board. Conversely, other games, like Dragonfly (7×7), specify that the king still castles two squares in each direction, and the rook is the piece that moves differently. A few variants, most notably Wildebeest chess (11×10), allow the player to choose how many squares he/she wants to move the king (and move the rook accordingly). Finally, there are also many variants, such as Grand chess, that do not allow castling.
In variants played on boards of different shapes (e.g. triangular, hexagonal, or three-dimensional), castling usually does not feature.
Chess without castling.
Writing in 2019, former world chess champion Vladimir Kramnik proposed a variant of chess where players would not have the ability to castle. As the king could no longer castle into safety, this variant would theoretically lead to more dynamic games, as it would be considerably harder to force a draw and the pieces would be forced to engage in a mêlée.
Regional variants.
Castling is a feature unique to Western chess. Regional variants of the game (such as "makruk, shogi, xiangqi, janggi" and "sittuyin") typically do not feature any moves that resemble castling, or indeed any moves that involve more than one piece moving. "Shogi" players often use the term "castle" to refer to the concept of building a multi-piece defensive structure that defends the king.
Notable examples.
Averbakh game.
In this game, Yuri Averbakh–Cecil Purdy (1960), Black castled queenside. Averbakh pointed out that the rook passed over a square controlled by White and thought it was illegal. Purdy pointed out that the castling was legal since this applies only to the king, to which Averbakh replied "Only the king? Not the rook?" , .
Edward Lasker game.
In this game, Edward Lasker–Sir George Thomas (London 1912), Black had just played 17...Kg1. White could have checkmated by 18.0-0-0, but instead played 18.Kd2#. (See Edward Lasker's notable games.)
Prins versus Day.
This game, Lodewijk Prins–Lawrence Day (1968), had ended earlier on move 28, but if Prins had played on he would have been unable to avoid this position. Black now checkmates by castling: 31...0-0-0. (See Lawrence Day's notable chess games.)
Feuer versus O'Kelly.
In the game Feuer–O'Kelly, Belgian Championship 1934, Feuer perpetrated what later became known as a famous opening trap against O'Kelly when he castled queenside with check, simultaneously attacking and winning O'Kelly's rook on b2, which had captured Feuer's pawn on that square.
History.
Castling has its roots in the "king's leap". There were two forms of the leap: (1) the king would move once like a knight, and (2) the king would move two squares on his 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 moved to a safe square by a two-step procedure: (1) the king moved to the second , and (2) the rook moved to the king's original square and the king moved to the rook's original square .
Before the bishop and queen acquired their current moves in the 16th century, they were weak pieces and the king was relatively safe in the middle of the board. When the bishop and queen got their current moves they became very powerful and the king was no longer safe on its original square, since it can be attacked from a distance and from both sides. Castling was added to allow the king to get to a safer location and to allow rooks to get into the game earlier .
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, if no capture was made, the king was not in check, and did not move over check. (The black king might move similarly.) In Lombardy, the white king might jump an additional square to b1 or h1 or to a2 (and equivalent squares for the black king). Later in Germany and Italy, the king move was combined with a pawn move.
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 Göttingen manuscript (c. 1500) and a game published by Luis Ramírez de Lucena in 1498, castling consisted of two moves: first the rook and then the king.
The current version of castling was established in France in 1620 and England in 1640 .
In the 1811 edition of his chess treatise, Johann Allgaier introduced the 0-0 notation. He differentiated between "0-0r" (r=right) and "0-0l" (l=left). The 0-0-0 notation for queenside castling was added in 1837 by Aaron Alexandre. The practice was then accepted in the first edition (1843) of the "Handbuch des Schachspiels".
Nomenclature.
In most European languages, castling is known by a derivative from the same root as the English word "rook" (e.g. "rochieren, rochada, enroque"), while the local adjectives meaning "long" and "short" (or "big" and "small") are used in those countries to refer to queenside and kingside castling, respectively.