Chess

Overview
Chess is a popular two-player abstract strategy board game which is of Indian and Persian descent.

Objective
The objective of Chess is to attack the opponent's King in such a way that the opponent cannot escape from the attack and cannot block it on his next turn.

Pieces

At the beginning of the game the first player has 16 light pieces and the second player has 16 dark pieces:

 

Light pieces Dark pieces
- one King - one King
- one Queen - one Queen
- two rooks - two rooks
- two bishops - two bishops
- two knights - two knights
- eight pawns - eight pawns

 

Board

Chess is played on an 8x8 square board.

The initial position of the pieces is shown in the following picture:

Play

Players move alternately, starting with the player controlling the white pieces.

No piece can be moved to a square occupied by another piece of the same color.

If a piece moves to a square occupied by an enemy piece the latter is considered to be captured and is removed from the board.  Capturing is not mandatory, i.e. if one of the player's pieces can capture an enemy piece it's not required to do so.

If a piece A can capture an enemy piece B it's said that A attacks B or that B is under attack.

The King is said to be "in check" if it is under attack by an enemy piece (even if the attacking piece cannot move for some reason).

A player may never leave his king "in check" at the end of his move.  The "in check" situation can be eliminated in one of the following ways:

  • The king can be moved to a square that is not under attack.
  • The attacking enemy piece can be captured by one of the player's pieces (even by the king, if doing so does not put the king in check).
  • The attack can be blocked by placing a piece between the king and the attacking enemy piece (this is not possible if the attacking piece is a knight).

All possible moves for each type of piece are explained below.


The Bishop can move any number of empty squares in any diagonal direction.  The bishop cannot jump over other pieces.


The Rook can move any number of empty squares horizontally or vertically.  The rook cannot jump over other pieces.  The rook is also moved when castling.


The Queen can move any number of empty squares diagonally, horizontally, or vertically.  The queen cannot jump over other pieces.


The Knight can move to the nearest square that is not on the same row, column or diagonal.  In other words the knight moves two squares horizontally or vertically and then one square perpendicular to that.  The Knight is the only piece that can jump over other pieces.


The Pawn can make several kind of moves and it captures enemy pieces in a different way from the other pieces:

  • a pawn can move one square forward on the same vertical column if that square is empty;
  • a pawn  can move two squares forward from its initial position if both squares are empty;
  • a pawn  can capture an enemy piece on any of the two squares diagonally adjacent to it and adjacent to the square in front of the pawn (i.e., the two adjacent squares diagonally in front of the pawn);

  • if an enemy pawn moves two squares forward from its initial position and lands on a square horizontally adjacent to a player's pawn then the player's pawn can capture the enemy pawn "en passant" as if it moved forward only one square rather than two, but only on the immediately subsequent move:

  • If a pawn reaches the last row then it is promoted to a queen, rook, bishop, or knight of the same color.  The player can choose any of the mentioned variants but, in practice, the pawn is almost always promoted to a queen.

The King can make two kinds of move:

  • it can move to any orthogonally or diagonally adjacent unoccupied square that is not under attack.  It is considered that these squares are under attack even if the attacking piece cannot move for some reason:

  • Once per game the king can make a special move called "castling".  Castling involves moving the king two squares towards a rook and then placing the rook on the other side of the king, adjacent to it:

The top left picture shows a position before the long castling for black and before the short castling for white.

The top right picture shows the position after castling.

The bottom left picture shows a position before the short castling for black and before the long castling for white.

The bottom right picture shows a position after the castling.

  •  Castling is only allowed if both a rook and the king meet the following conditions:
  1. The king and the corresponding rook have never moved during the game;
  2. No pieces are standing between the king and the rook;
  3. The king may not currently be in check, nor may the king pass through or land on a square that is under attack by one or more enemy pieces (though the rook is permitted to be under attack).
  4. The rook must be the original one and not a pawn promoted to a rook.

 

End of Game

If a player puts an enemy king in check and the opponent cannot eliminate it on his next move, then the game ends and the player wins.  Such a position is called checkmate.  In the following picture the black king is in check (under attack of the white rook) and there is no way for black to escape from this situation:

The game ends in a draw if a player's king is not in check and he has no legal moves on his turn.  I.e. the player has no other pieces (or all of his pieces cannot move) and he cannot move his king without putting him in check.  Such a situation is called a stalemate.  In the following picture the black king is not in check but he cannot move since each of the two unoccupied adjacent squares are under attack by the white rook:

The game ends in a draw if there is no possibility for either player to checkmate the opponent.  For example one player has a king and a knight and another only a king.

Any player can claim a draw if one of the following conditions exists:

  • No pawn has been moved and no piece has been captured during fifty subsequent moves played by each player.
  • The same board position has been repeated three times.  The positions are considered to be identical if it is the turn of the same player and all pieces have the same rights to move, including the right to castle or capture en passant.

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