There are some exceptions to the basic rules regarding captures. A pawn can not capture pieces positioned on the square(s) the pawn can advance to; instead it may capture an enemy piece that is placed one square diagonally and in front of it. For example, suppose WHITE has a pawn located at e2. The pawn can move both to e3 and e4 if no piece interferes, but it can not capture on these two squares. Instead it can capture on d3 or f3.
A special case of pawn capture exists when moving the pawn two squares forward (remember this is only possible if the pawn is still on its initial position). In this case an enemy pawn that would have been able to capture it, had it moved only one square instead of two, is still able to capture it. This capture is called 'en passant' and it is only valid for one move : if the player who may capture 'en passant' does not use this right in his next move, he will no longer be able to do it later. For example, if WHITE moves his e2-pawn (Pe2) to e4 (we write this move as 'e2-e4' or simply 'e4') and there is a black pawn at d4 (Pd4), then BLACK is able to capture en-passant ('Pd4xe3 e.p.' or simply 'dxe3'), just as if the white pawn had moved to e3. But if he decides to play something else, he no longer retains this right.
A King can not be captured. In effect, the King can not move to a square controlled by an enemy piece; so there never arises such a case. We say a square is 'controlled by a piece' if the piece can capture on this square. Also note that, regarding a Knight's move, there are no interfering friendly or enemy pieces; the Knight can always land on its destination square.
A pawn that manages to traverse all the way to the other side of the chess board is 'promoted'. When a promotion occurs the player replaces the pawn with a same-coloured piece of his choice (King and Pawn excluded). Usually these pawns are promoted to Queens.
The King also has a special move at his disposal. It is called 'castling' and it is a defensive move, used to increase the King's safety. Castling may be either 'short' or 'long'. Consider the white King on e1 (Ke1) and the white Rook on h1 (Rh1). 'Short castling' involves moving the King to g1 and simultaneously the Rook to f1. The whole procedure is considered to be one single move, not two (we denote short castling by 'O-O'). However, there are some constraints before one can castle : both the King and the Rook must not have moved before, all in-between squares must be empty and both landing squares (f1 and g1 in this example) must not be threatened by enemy pieces. In addition, the King must not be in check. If all these chess rules apply, one may castle. Long castling (denoted by 'O-O-O') is similar, the only difference being that the a-Rook is used instead. The ending squares are c1 for the King and d1 for the Rook. All the same applies for BLACK too, the corresponding squares being g8 and f8 for short castling and c8 and d8 for long castling.
Chess games always start with WHITE moveing and players taking alternating turns. Moving a piece is obligatory, one cannot 'pass'. The player who checkmates his opponent is the winner. There can also be a draw. This happens when no side has enough control to checkmate their opponent. If, for example, WHITE is left with the King and a Bishop and BLACK with only his King, there is no way that WHITE can checkmate BLACK and the game is drawn. A draw may also be agreed at any time. Other draw cases include the 'triple repetition' of the same position, the '50 moves rule' (game is drawn if no pawn advance or capture has taken place during the last 50 moves) and 'stalemate'. A player is stalemated if he has no valid moves, but he also is not in check. Recall that when one is in check and has no valid moves, he is 'checkmated' and he loses the game. Finally, a player who is able to check his opponent for as long as he wishes, may claim draw by 'continuous check', a special case of the 50 moves rule.
