Castling
The most common of the three special chess rules is called castling—a move that is normally used to improve the king’s safety. Castling is the only move that allows two pieces, the king and a rook to move at the same time. You can only castle if all of the following conditions are present:
If neither the king nor the rook being used has been moved yet during the game, castling is possible. If either piece has been moved, then castling is not allowed, even if the piece is moved back to its original square.All of the squares between the king and the rook must be empty to the castle.The king must not be in check when castling is performed, nor can castling move the king through a square where it would be in check.
If all of these conditions are met, you can castle: This may sound confusing, but in practice it is simple. In the diagram above, the white king and rooks are positioned where they should be before castling. The black kings and rooks are positioned properly after castling. Castling kingside is more common and leaves the king on the g-file while the rook moves to the f-file. Castling queenside leaves the king on the c-file, while the rook moves to the d-file. Files, in chess, are columns designated by a letter, as displayed in the diagram.
Pawn Promotion
Pawns are the weakest pieces on the board, but they have the potential to become much stronger. Should a pawn manage to make it all the way to the other end of the board, that pawn must be promoted to any piece its player chooses, other than a king. Generally, you would promote a pawn to a queen; however, you can also promote it to a rook, knight, or bishop. When the pawn is promoted to a queen, the move is often termed queening, and it is allowable for there to be two queens of the same color on the board. Sometimes a rook is used in an inverted position to designate the second queen. Promoting to something other than a queen is known as underpromotion. Since the queen is the most powerful piece, promotion is for a queen. However, there may be a rare instance where the movement of a knight or other piece might offer an immediate benefit, in which case underpromotion is used. The diagram shows a pawn from each side preparing to promote.
En Passant
En passant—a French term that means “in passing”—is probably the most confusing move for novice chess players. Players may not even know the move exists, making it the source of many arguments. Before the 15th century, most people played by rules that didn’t allow pawns to move two squares on their first move. When the two-square-pawn move was added to speed up the opening phase of the game, players noticed that the pawn could now sneak by an enemy pawn on an adjacent—something that was never possible when pawns plodded along at one square per move. The solution was en passant, a move that allows a pawn that has moved two squares to be captured as though it had only moved one. The diagram illustrates how en passant works. The following conditions must all be present for an en passant capture to be legal:
The capturing pawn must be on its fifth rank.The opponent must move a pawn two squares, landing the pawn directly alongside the capturing pawn on the fifth rank.You must make the capture immediately; you only get one chance to capture en passant.
If all those conditions are met, an en passant capture is possible. In the diagram, Black’s pawn has just moved from c7 to c5, landing it directly next to White’s pawn on d5. If White wishes, he may capture Black’s pawn by moving his pawn to c6, capturing the pawn as though it had only moved one square. However, if he chooses not to capture immediately, White loses this option. The diagram also shows a second example from Black’s perspective. White has just moved a pawn from f2 to f4. Black’s pawn on g4 may capture White’s pawn by moving to f3 on the next turn. If Black chooses not to make this capture, he loses the ability to capture en passant.
