Special moves
a. Castling
Once in every game, each king is allowed to make a special move, known as castling. Castling consists of moving the king two squares towards a rook, then placing the rook immediately on the far side of the king. Castling is only permissible if all of the following conditions hold:
Neither of the pieces involved in the castling may have been previously moved during the game;
There must be no pieces between the king and the rook;
The king may not currently be in check, nor may the king pass through squares that are under
attack by enemy pieces. As with any move, castling is illegal if it would place the king in check.
b. En passant
When a pawn advances two squares, if there is an opponent's pawn on an adjacent file next to its destination square, then the opponent's pawn can capture it and move to square the pawn passed over, but only on the next move. For example, if the black pawn on g5 has just advanced two squares to g5, then one of the white pawns can take it via en passant on g6.
c. Promotion
When a pawn advances to its eighth rank, it is exchanged for the player's choice of a queen, rook, bishop, or knight of the same color. Usually, the pawn is chosen to be promoted to a queen, but in some cases another piece is chosen, called underpromotion. In the diagram on the right, the pawn on c7 can choose to advance to the eighth rank to promote to a better piece.
d. Game's end
Chess games do not have to end in checkmate — either player may resign if the situation looks hopeless. Games also may end in a draw (tie). A draw can occur in several situations, including draw by agreement, stalemate, threefold repetition of a position, the fifty move rule, or a draw by impossibility of checkmate (usually because of insufficient material to checkmate).
e. Time control
Besides casual games without exact timing, chess is also played with a time control, mostly by club and professional players. If a player's time runs out before the game is completed, the game is automatically lost. The timing ranges from long games played up to seven hours to shorter rapid chess games lasting usually 30 minutes or one hour per game. Even shorter is blitz chess with a time control of three to fifteen minutes for each player and bullet chess (under three minutes).