Article 3: The moves of the pieces
3.1 It is not permitted to move a piece to a square occupied by a piece of the same colour.
3.1.1 If a piece moves to a square occupied by an opponent’s piece the latter is captured and removed from the chessboard as part of the same move.
3.1.2 A piece is said to attack an opponent’s piece if the piece could make a capture on that square according to Articles 3.2 to 3.8.
3.1.3 A piece is considered to attack a square even if this piece is constrained from moving to that square because it would then leave or place the king of its own colour under attack.
3.2 The bishop may move to any square along a diagonal on which it stands.
3.3 The rook may move to any square along the file or the rank on which it stands.
3.4 The queen may move to any square along the file, the rank or a diagonal on which it stands.
3.5 When making these moves, the bishop, rook or queen may not move over any intervening pieces.
3.6 The knight may move to one of the squares nearest to that on which it stands but not on the same rank, file or diagonal.
3.7.1 The pawn may move forward to the square immediately in front of it on the same file, provided that this square is unoccupied, or
3.7.2 on its first move the pawn may move as in 3.7.1 or alternatively it may advance two squares along the same file, provided that both squares are unoccupied, or
3.7.3 the pawn may move to a square occupied by an opponent’s piece diagonally in front of it on an adjacent file, capturing that piece.
18.104.22.168 A pawn occupying a square on the same rank as and on an adjacent file to an opponent’s pawn which has just advanced two squares in one move from its original square may capture this opponent’s pawn as though the latter had been moved only one square.
22.214.171.124 This capture is only legal on the move following this advance and is called an ‘en passant’ capture.
126.96.36.199 When a player, having the move, plays a pawn to the rank furthest from its starting position, he must exchange that pawn as part of the same move for a new queen, rook, bishop or knight of the same colour on the intended square of arrival. This is called the square of ‘promotion’.
188.8.131.52 The player's choice is not restricted to pieces that have been captured previously.
184.108.40.206 This exchange of a pawn for another piece is called promotion, and the effect of the new piece is immediate.
3.8 There are two different ways of moving the king:
3.8.1 by moving to an adjoining square
3.8.2 by ‘castling’. This is a move of the king and either rook of the same colour along the player’s first rank, counting as a single move of the king and executed as follows: the king is transferred from its original square two squares towards the rook on its original square, then that rook is transferred to the square the king has just crossed.
Before white kingside castlingBefore black queenside castling
After white kingside castlingAfter black queenside castling
Before white queenside castlingBefore black kingside castling
After white queenside castlingAfter black kingside castling
220.127.116.11 The right to castle has been lost:
18.104.22.168.1 if the king has already moved, or
22.214.171.124.2.with a rook that has already moved.
126.96.36.199 Castling is prevented temporarily:
188.8.131.52.1 if the square on which the king stands, or the square which it must cross, or the square which it is to occupy, is attacked by one or more of the opponent's pieces, or
184.108.40.206.2 if there is any piece between the king and the rook with which castling is to be effected.
3.9.1 The king is said to be 'in check' if it is attacked by one or more of the opponent's pieces, even if such pieces are constrained from moving to the square occupied by the king because they would then leave or place their own king in check.
3.9.2 No piece can be moved that will either expose the king of the same colour to check or leave that king in check.
3.10.1 A move is illegal when it fails to meet the relevant requirements of Articles 3.1 - 3.9
3.10.2 A move is legal when all the relevant requirements of Articles 3.1 - 3.9 have been fulfilled.
3.10.3 A position is illegal when it cannot have been reached by any series of legal moves.