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2 Castling
2.1 Rules for Castling
Once the starting position is set up, the rules for play are the same as standard chess. In particular, pieces and pawns have their normal moves, and each player's objective is to checkmate their opponent's king.
Fischer random chess allows each player to castle once per game, a move by potentially both the king and rook in a single move. However, a few interpretations of standard chess games rules are needed for castling, because the standard rules presume initial locations of the rook and king that are often untrue in Fischer Random Chess games.
After castling, the rook and king's final positions are exactly the same positions as they would be in standard chess. Thus, after a-side castling (notated as O-O-O and known as queen-side castling in orthodox chess), the King is on c (c1 for White and c8 for Black) and the Rook is on d (d1 for White and d8 for Black). After h-side castling (notated as O-O and known as king-side castling in orthodox chess), the King is on g and the Rook is on f. It is recommended that a player state "I am about to castle" before castling, to eliminate potential misunderstanding.
However, castling may only occur under the following conditions, which are extensions of the standard rules for castling:
1. Unmoved: The king and the castling rook must not have moved before in the game, including castling.
2. Unattacked: All of the squares between the king's initial and final squares (including the initial and final squares) must not be under attack by any opposing piece.
3. Vacant: All the squares between the king's initial and final squares (including the final square), and all of the squares between the rook's initial and final squares (including the final square), must be vacant except for the king and castling rook.
These rules have the following consequences:
* If the initial position happens to be the standard chess initial position, these castling rules have exactly the same effect as the standard chess castling rules.
* All the squares between the king and castling rook must be vacant.
* Castling cannot capture any pieces.
* The king and castling rook cannot "jump" over any pieces other than each other.
* A player may castle at most once in a game.
* If a player moves his king or both of his initial rooks without castling, he may not castle during the rest of the game.
* In some starting positions, some squares can stay filled during castling that would have to be vacant in standard chess. For example, after a-side castling (O-O-O), it's possible for to have a, b, and/or e still filled, and after h-side castling (O-O), it's possible to have e and/or h filled.
* In some starting positions, the king or rook (but not both) do not move during castling.
* The king may not be in check before or after castling.
* The king cannot move through check.
2.2 How to Castle
When castling on a physical board with a human player, it is recommended that the king be moved outside the playing surface next to his final position, the rook then be moved from its starting to ending position, and then the king be placed on his final square. This is always unambiguous, and is a simple rule to follow.
Eric van Reem suggests that there are other acceptable ways to castle:
* If only the rook needs to move (jumping over the king), you can simply move only the rook.
* If only the king needs to move (jumping over the castling rook), you can simply move the king.
* You can pick up both the king and rook (in either order), then place them on their final squares (this is called "transposition" castling).
* You can move the king to its final square and move the rook to its final square as two separate moves, in either order (this is called "double-move" castling). Obviously, if the rook is on the square the king will occupy you'll need to move the rook first, and if the king is on the square the rook will occupy you'll need to move the king first.
In contrast, Reinhard Scharnagl strongly recommends that, since castling is fundamentally a king's move, the king should always move first.
Generally, when playing with human player on a physical board, it's wise to announce "I'm going to castle" before castling. If you're playing a timed game, once you're done castling press the appropriate button on your chess clock to show your move has completed.
When castling using a computer interface, programs should have separate a-side (O-O-O) and h-side (O-O) castling actions (e.g., as a button or menu item). Ideally, programs should also be able to detect a king or rook move that cannot be anything other than a castling move and consider that a castling move.
When using an electronic board, to castle you should remove the king, remove the castling rook, place the castling rook on its new position, and then place the king on its new position. This will creates an unambiguous move for electronic boards, which often only have sensors that can detect the presence or absence of an object on each square (and cannot tell what object is on the square). Ideally, electronic boards should detect a king or rook move that can only be a castling move as well, but users should not count on this.
2.3 Castling Rule Ambiguities
Many published castling rules are unfortunately ambiguous. For example, the rules first published by Eric van Reem and chessvariants.com, as literally stated, did not specifically state that there must be vacant squares between the king and his destination except for the participating rook. As a result, those rules appeared to some to allow the king to "leap" over other pieces.
In 2003 David A. Wheeler contacted many active in Fischer Random Chess to determine the exact castling rules, including Eric van Reem, Hans-Walter Schmitt, and R. Scharnagl. All agreed that there must be vacant squares between the king and his destination except for the participating rook, clarifying the castling rules.