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Castling Rules in Chess Variants

by Greg Strong


Castling is a special move in chess where a player moves both his king and one of his rooks as part of the same move. It both develops the rook and moves the king toward the corner for better safety. It is the only move in orthodox chess where a player moves two of his pieces in the same turn.

Castling in Orthodox Chess

A player may make a castling move with a king and a rook if a number of conditions are met:

When castling, the king moves two spaces in the direction of the rook it is castling with, and the rook "jumps" to the square immediately on the other side:

Castling with the closer rook is called castling "short" or castling "king-side" and is annotated as O-O. Castling with the farther rook is called castling "long" or "queen-side" and is annotated O-O-O.

Side Castling King Start King End Rook Start Rook End
white short e1 g1 h1 f1
white long e1 c1 a1 d1
black short e8 g8 h8 f8
black long e8 c8 a8 d8

History of Castling

In early forms of chess, such as Shatranj, there were no diagonal sliders. The only sliding pieces were the rooks and they were slow to develop, so the king was not terribly endangered in the center. With the addition of the modern bishop and queen, however, the king’s starting location became more perilous. An early attempt to address this was known as the king's leap. With this move, an unmoved king could make a leap - typically either two spaces or a knight's move (rules varied in different times and places).

Our modern castling rule dates to the 1600s in France and England. In Rome, however, a variant known as "free castling" was used until the late 1800s. In free castling, the king could move as many spaces toward the rook as desired, including all the way into the corner, and the rook could move to any square passed over, including the king's starting square, so long as the move did not give check to the enemy king. A few players have advocated bringing back this rule to make the game more dynamic without radically altering its character.

Castling in Chess Variants

The main alternatives to Western Chess, Xiangqi (Chinese Chess) and Shogi (Japanese Chess), do not have a castling rule, despite having kings and rooks (although, in Xiangqi, the king cannot leave the 3x3 castle).

Chess variants that are played on the standard board with the standard pieces and setup usually keep the castling rule as-is (e.g., Berolina Chess, Extinction Chess). Cylindrical Chess, on the other hand, has seen variety on how castling is implemented, although it could be argued that this game does not use the standard board, since it is conceptually cylindrical.

Some variants use the standard board and pieces but in random arrangements ("shuffle" variants). These deal with castling in different ways described on their respective pages. See Fischer Random Chess and Chess480 as examples of games that are otherwise identical except for how they have adapted the castling rule.

In some games, the concept of a castling move is preserved, but utilizing different pieces. In Knightmate, for example, the royal piece is a knight, and it is the knight that castles (although still with a rook.) Alternatively, in Chess with Different Armies, the king still castles, but often with a piece other than a rook (depending on the army in play.) With the Colorbound Clobberers army, the rook replacement is colorbound, requring a further modification - when castling long, the king moves three spaces instead of two so that the square color of the castling piece doesn't change.

Chess variants have been made by changing the geometry of the board. Orthodox chess has been adapted to circular, hexagonal, diamond, and three-dimensional boards, among others. Such games almost never have castling. In some cases, it could be that the geometry offered more mobility and a castling rule didn’t seem necessary. In other cases, it may have been that there was no obvious adaption, and any potential castling rule would seem too forced.

Many variants have been made by enlarging the board and adding extra pieces. Given a wider board, some modification to the standard castling rule is required. This is typically addressed in one of three ways:

Modern Innovations

Flexible Castling - this innovation, by Fergus Duniho, is a more limited form of free castling that has since been used in many games by Duniho and others. In flexible castling, the king may move two or more squares towards the rook, but must stop before the rook's square. The rook then leaps over to the square immediately on the other side.

When multiple castlings in the same direction are possible, the O-O / O-O-O notation would no longer be unambiguous. In its original form flexible castling is uniquely determined by the destination of the king, and can simply be written as a normal king move, such as e1-b1 or Kb1. If the king was allowed to also move only one square on castling, as in Wildebeest Chess, then the destination would not distinguish between a castling move and a normal king move in this case. To resolve that, the castling can use a tilde between the destination and the piece or square of origin, like K~d1 or e1~d1, to distinguish it from the normal move. This notation could be used even in cases where there is no ambiguity, to stress the fact that one is dealing with a castling (as the O-O notation also does).

Fast Castling - for his game, Wide Chess, Kevin Pacey created "fast castling". By this rule, an unmoved king who is not in check may leap to any unattacked empty square in the direction of an unmoved rook, and the rook then leaps to the king's original square. The squares in between do not need to be empty.

XBetza Notation

Prolific chess variant inventor Raplh Betza invented a simple notation, typically called Betza Notation, for describing the move capabilities of fairy chess pieces. This notation is useful but is not sufficient for pieces with strange capabilities or things such as castling. An extended notation, known as XBetza Notation, has been developed by H. G. Muller, with feedback from the community, to address additional use cases.

In XBetza notation, the ability to castle is indicated by the atom 'O' (inspired by the O-O notation). A following digit must indicate the King displacement. Usually the O will be modified by both 'i', which indicates the piece must not have moved before, and 's' to indicate that only sideways moves are allowed. So, castling in orthodox chess would be described "isO2" in XBetza notation. Castling occurs with the piece on the farthest square in the specified direction, although an additional 'j' modifier will indicate that the castling piece is one square closer.

For flexible castling, or other situations where more than one option is possible, each option must be listed separately. A king that can move two or three squares to castle would be "isO2isO3". It is sometimes the case that the king can move farther in one direction than the other, requiring the 'l' or 'r' modifier (for left or right.) Therefore, for Schoolbook Chess as an example, where the king can move two or three spaces to the right, but up to four to the left, the notation for the white king would be "isO2isO3ilO4". Since the left and right modifiers are always from the first player's point of view, however, the notation for the black king would be "isO2isO3irO4".

Fast castling is denoted by pO, which is a (non-standard) use of the 'p' modifier to indicate pieces may be (but not 'must be') hopped over in the process. In this case the range is interpreted as the maximum distance the King can move.

Castling on Boards with 10 Files

Chess variants on 10x8 and 10x10 boards are especially common. Given this, when making the variant chess program ChessV, Greg Strong identified the various implementations of castling so that users could add new variants and simply select the desired castling rule.

Castling Type Description Example
None No castling Carrera's Chess
Standard King starting on e or f file slides three squares either direction, subject to the usual restrictions, to castle with the piece in the corner Capablanca's Chess
Long King starting on e or f file slides three squares when castling short or four when castling long, subject to the usual restrictions, to castle with the piece in the corner Janus Chess
Close-Rook King starting on e or f file slides two squares either direction, subject to the usual restrictions, to castle with the piece on the b or i file Victorian Chess
Flexible King starting on e or f file slides two or more squares, but stopping short of the corner, subject to the usual restrictions, to castle with the corner piece Grotesque Chess
Close-Rook Flexible King starting on e or f file slides two or more squares, subject to the usual restrictions, to castle with the piece on the b or i file none
Free King starting on e or f file slides any number of squares, including all the way into the corner, subject to the usual restrictions, to castle with the corner piece, which may move onto any square the king passed over, including its starting square Aberg's Capablanca Variant
Fast Unmoved king starting on e or f file leaps any number of squares towards an unmoved corner piece to any square that is not attacked, and the corner piece leaps to the king's starting square Waffle Chess