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Castling

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Initial position of kings and rooks. Kings may be moved to the indicated squares.
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White has castled kingside (0-0) and Black has castled queenside (0-0-0).

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 rank, then moving the rook to the square over which the king crossed. Castling may only be done 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 in which it would be in check. Castling is one of the rules of chess and is technically a king move (Hooper & Whyld 1992:71).

The notation for castling, in both the descriptive and the algebraic systems, is 0-0 with the kingside rook and 0-0-0 with the queenside rook; in PGN, O-O and O-O-O are used instead. Castling on the kingside is sometimes called castling short and castling on the queenside is called castling long - the difference based on whether the rook moves a short distance (two squares) or a long distance (three squares) (Hooper & Whyld 1992).

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

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Black may not castle on either side because he is in check from the white queen.
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White to move may not castle kingside because the black queen controls g1. White may castle queenside, however, even though the a1-rook is under attack.

Castling is permissible if and only if all of the following conditions hold (Schiller 2001:19):

  1. The king and the chosen rook are on the player's first rank.
  2. Neither the king nor the chosen rook has previously moved.
  3. There are no pieces between the king and the chosen rook.
  4. The king is not currently in check.
  5. The king does not pass through a square that is attacked by an enemy piece.
  6. The king does not end up in check. (True of any legal move.)

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 misperception 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 moves the king into a safer position away from the center of the board, and it moves the rook to a more active position in the center of the board (it is even possible to checkmate with castling).

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 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-file is undefended; the king is thus often moved to the b-file 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; 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 efficient 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.

Tournament rules

Under the strict touch-move rules enforced in most tournaments, castling is considered a king move. But under current US Chess Federation rules, a player who intends to castle and touches the rook first would suffer no penalty, and would be permitted to perform castling, provided castling is legal in the position. Still, the correct way to castle is to first move the king. As usual, the player's mind may change between all legal destination squares for the king until it is released. When the two-square king move is completed, however, the player has formally chosen to castle (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 (Just & Burg 2003:13-14,17-18,23).

It is also required by the official rules 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 (Just & Burg 2003:13-14,17-18,23).

The right to castle must be the same in all three positions for a valid draw claim under the threefold repetition rule.

Chess variants and problems

Some chess variants, for example Chess960, have modified castling rules to handle modified starting positions. Castling can also be adapted to large chess variants, like Capablanca chess, which is played on a 10x8 board.

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 chosen rook has previously moved.

Notable examples

Averbakh vs. Purdy, 1960
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Black to move, castled queenside, with the rook going over the attacked square b8.

Averbakh game

In this game between Yuri Averbakh and Cecil Purdy, Black castled queenside. Averbakh pointed out that the rook passed over a square controlled by White and thought it was illegal. Purdy proved that the castling was legal since this applies only to the king, to which Averbakh replied "Only the king? Not the rook?" (Evans 1970:38-39), (Lombardy & Daniels 1975:188).

Ed. Lasker vs. Thomas, 1912
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Position after 17...Kg1

Lasker game

In this game between Edward Lasker and Sir George Thomas (London 1912), Black had just played 17...Kg1. White might have checkmated by 18.0-0-0# but instead played 18.Kd2#. (See Edward Lasker's notable games.)

Prins vs. Day, 1968
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Position before 31...0-0-0#

Prins versus Day

This game between Lodewijk Prins and Lawrence Day ended in a checkmate by castling: 31...0-0-0#. (See Lawrence Day' notable chess games.)

Feuer vs. O'Kelly, 1934
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Position after 12...Kxd8

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 rank and (2) the rook moved to the king's original square and the king moved to the rook's original square (Davidson 1981:48).

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 (Davidson 1981:16).

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 (Sunnucks 1970:66).

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

Castling is in most European languages other than English known as "roszada", "rochieren", "rochada", "rocada", "enroc", "enroque", "rošáda", "arrocco", "rokada", "рокировка", or some other derivative of the same root (from which also the English word "rook" is derived), 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.

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