In most chess games one side or the other must gain an advantage in strength, called a material advantage, before mounting an attack that leads to checkmate. Even the advantage of a single pawn may be enough to lead to a win, for the pawn may promote and become a queen. In comparing the values of chessmen, a pawn is normally valued as 1, a knight or bishop 3, a rook 5, and a queen 9. The king's worth cannot be evaluated because it cannot be lost without losing the game. In the end game, however, when there are few pieces left on the board, the king often becomes an active fighting piece. It may, if more actively engaged than the opposing king, determine victory in the end game. A bishop is worth slightly more than a knight, except in positions where many pawns remain on the board and their advance is blocked. In such positions the knight can jump over the interlocked pawns, but the bishop may be blocked. Two rooks are usually worth more than a queen.
A queen, with the aid of the king, can checkmate a lone king. So can a rook, two bishops, or a knight and a bishop. A single bishop, single knight, or two knights is not sufficient force to checkmate, except in certain positions where the opposing king is confined by his own men, usually pawns.
Captures. All captures are optional. In capturing, a chessman removes an enemy chessman from the board and occupies its square. All chessmen except the pawn capture as they move. The pawn, however, may capture an enemy chessman on either of the squares diagonally in front of it. The pawn's advance is blocked by a chessman directly in front of it. In certain positions a pawn may capture an enemy pawn en passant (French for "in passing"). If, for example, a White pawn in its initial position is faced by a Black pawn two squares ahead on an adjacent file, the White pawn may move two squares forward, past the square where it would normally be open to capture by the enemy pawn. However, the Black pawn may capture en passant by moving diagonally forward and removing the White pawn as though it had stopped on the vulnerable square. This capture must be made on Black's next move or it may not be made at all.
Promotion. When a pawn reaches the last rank on the far side of the board, it is promoted to a queen, rook, bishop, or knight and immediately assumes its new powers. The choice is usually a queen, the most powerful piece, and is therefore often called queening. As a result of promotions one side may have several queens on the board at the same time. Because a chess set has only one queen in each color, an upside-down rook or a coin is sometimes used to represent an additional queen.
Castling. Once during a game each player's king and one of his rooks may move simultaneously. The move is called castling. The king is played two squares toward either rook, which then jumps over the king and occupies the adjoining square. Castling is possible only if neither the king nor the rook has moved and if the squares between the two pieces are vacant. The king must not at the time be under attack and must not move into or over a square that is attacked. Castling is a valuable means of bringing the rooks into play and, in some games, of protecting the king behind a row of unadvanced pawns.
Checkmate. A win in chess is accomplished by attacking the enemy king in such a way that it can no longer be defended. This is called a checkmate. The player whose king is attacked, or checked, must defend the monarch at any cost. He must move the king out of check, capture the attacker, or place another chessman between the attacker and the king. If none of these defenses is possible, the game is lost.
Drawn Games. Opponents may agree to a draw at any time. Games may also be drawn in the following ways:
Stalemate. A game is drawn automatically if a player whose king is not in check can make no move without putting the king in check.
Perpetual Check. A player may claim a draw if he demonstrates that he can continue to check the enemy king indefinitely.
Recurrent Position. A player may claim a draw if an identical position repeats three times, each time with the same side to move, and if he claims a draw before another move is made. He may also claim a draw if he first announces his intention and then makes a move that produces a position that has occurred twice before with the same side to move.
Fifty-Move Rule. A draw results if there have been no captures or pawn moves during 50 consecutive moves by each player. Either player claims the draw. '
Touch Rules. In tournament games, and by agreement of the players in nontournament games, a chessman that is touched must be moved. The only exceptions to this rule occur when the touched piece has no legal moves or when the player gives prior notice that he intends to adjust his chessmen. An enemy chessman that is touched must be captured if possible. When a player removes his hand from a piece he has played, the move cannot be taken back. In castling, the king should be moved before the rook.
Stages of the Game
Opening. The chessmen are moved forward from their original positions so that they become more useful for attack and defense. In the opening moves, players usually contend for control of the center of the board. From this position a player controls key squares on all sides, thus cramping his opponent's mobility.
Opening moves have been intensively analyzed by chess experts. Master chess players usually have favorite openings and are thoroughly familiar with most of the developments likely to occur during the first 15 or 20 moves of the opening variations in wliich they specialize.
Many openings and their variations have been named. Some honor great chess players, like the Ruy Lopez opening and the Reti opening. Others are named after countries, like the "Sicilian defense and the French defense. Still others describe something about the opening, like the center game and the queen's gambit.
Middle Game. In the middle game the players attack and defend from positions developed through the opening moves. Many pieces and pawns are still on the board, and the complications are immense. Identical middle-game positions rarely repeat from one game to another. Players must rely upon imagination, tactical ability, and analysis of future possibilities.
End Game. The end game is the stage at which only a few chessmen are left on the board. At this stage a player is sometimes able to force a win with only slight material or positional advantage. Much of endgame play attempts to promote pawns. The king can often become active without danger during the end game, attacking enemy pawns or guiding his own pawns toward the last rank.
Endings, like openings, have been intensively analyzed. Thousands have been summarized in books, and chess masters are likely to know many more.