In chess, a fork is a tactic that uses one piece to attack two or more of the opponent's pieces at the same time, hoping to achieve material gain (by capturing one of the opponent's pieces) because the opponent can only counter one of the two (or more) threats. The piece moving to make the multiple attack on the opponent's pieces is the forking piece. The opponent's pieces which are attacked by the forking piece are ones which are forked.
The type of fork is commonly named after the type of forking piece. For example, in a knight fork, a knight moves to attack two or more opponent's pieces in the same move. Any type of chess piece, including a king, may be able to become a forking piece. Any types of pieces can be forked, although a king cannot fork a piece that will attack it, because a king cannot move into check. The forking move itself may or may not be a move which captures an opponent's piece.
Knights are often used for forks: they jump to a position from where they attack two pieces.
Pawns can also fork enemy pieces: by moving a pawn forward, it may attack two pieces: one diagonally to the left and one diagonally to the right. In the diagram, the black pawn is forking the two white rooks. (Remember that by convention the board is oriented with Black's first row at the top, so the black pawn is moving downward.)
A queen move also often attacks two pieces at the same time, but this typically gains material only if both pieces are undefended, or if one is undefended and the other is the opposing king. Since the queen is usually more valuable than the pieces it is attacking, it usually only gains material capturing undefended pieces. However, the possibility of a queen fork is a very real threat when the queen is out in the open, as is often the case in an end game. A fork by a protected queen of the opposing queen and king (or an undefended piece) can be useful if the forking player wants to force an exchange of queens.
The term royal fork is sometimes used to describe the situation where the king and queen are forked, and when three or more pieces are attacked by a knight concomitantly the situation is sometimes referred to as a "family fork", especially (though not exclusively) if the king is one of the pieces so menaced.
The following example of a fork is from the first round of the FIDE World Chess Championship 2004 between Mohamed Tissir and Alexey Dreev. After White's 33rd move the following position was reached:
After 33...Nf2+ 34.Kg1 (the only legal move) 34...Nd3, White resigned. In the final position the black knight is forking the white queen and rook, so that after the queen moves away, White will lose the exchange (a rook for a minor piece).
These moves are given in algebraic chess notation.
Forks are often used as part of a combination which may involve other types of chess tactics as well.
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