1.Castling
There are a few rules in chess that allow pieces to do things beyond their basic movements.
The most common of these rules is castling, a move that is normally used to improve the king's safety. Castling is the only move that allows two pieces to move at the same time - the king and a rook.
Castling can only be done if the following conditions are all present:
- Neither the king nor the rook being used have been moved yet during the game. 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.
- The king must not be in check, nor can castling move the king through a square where he would be in check.
If all these conditions are met, castling is done by moving the king two squares towards the rook, while the rook hops over the king, landing on the square next to the king.
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.
2.Pawn Promotion
Pawns may be 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 promote to any piece its owner wants, other than a king. Most of the time, a pawn will be promoted to a queen; however, you can also choose to promote to a rook, knight or bishop. Promoting to something other than a queen is known as under promotion. The diagram above shows a pawn from each side preparing to promote.
3.En Passant
En passant (French for "in passing") is probably the most confusing move for novice chess players. In fact, many may not even know the move exists, making it the source of many arguments.
Why does such a confusing rule exist? Before the 15th century, most people played by rules which didn't allow the 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, it was noticed that it was now possible for a pawn to sneak by an enemy pawn on an adjacent file -- 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 which has moved two squares to be captured as though it only moved one.
The diagram above 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.
- The capture must be made 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 above, 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 above 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 very next turn. If Black chooses not to make this capture, he loses the ability to capture en passant.
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