Special Moves
Castling
Castling is a special move in the game of chess involving the king and either of the original rooks of the same color. Castling consists of moving the king two squares towards a rook, then moving the rook onto the square over which the king crossed.
Castling is considered a King move. Castling is permissible only if all of the following conditions hold:
Pawn Promotion
If a Pawn advances all the way to its eigth rank, it is then promoted to a Queen, Rook, bishop, or Knight of the same color, the choice being at the discretion of its player. The choice is not limited to previously captured pieces.
Check
When a player makes a move that threatens the opposing King with capture (not necessarily by the piece that was moved), the king is said to be in check. If a player's king is in check then the player must make a move that eliminates the threat(s) of capture; a player may never leave his or her king in check at the end of his or her move.
The possible ways to remove the threat of capture are:
In the case of double check, when there are two pieces attacking the King, only a King move will get out of check; the King may capture an enemy piece. A player may not make any move which places or leaves his or her King in check, even if the checking piece cannot move due to a pin, i.e. moving it would expose his or her own King to check. This also means that a player cannot place his King on any square adjacent to the enemy King, because doing so would leave his or her King able to be taken by the enemy King and therefore in check.
Checkmate
If a player's King is placed in check and there is no legal move that player can make to escape check, then the King is said to be checkmated, the game ends, and the checkmated player loses. Unlike other pieces, the king is never actually captured or removed from the board because checkmate ends the game.