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 piece may perform a fork, including a king, and any type of piece can be forked.
Knights are often used for forks; they jump to a position where they simultaneously attack two pieces and cannot be counterattacked.
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 endgame. 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.
Pawns can also fork enemy pieces: by moving a pawn forward, it may attack two pieces: one diagonally to the left and the other diagonally to the right. In the diagram to the right, the black pawn is forking the two white rooks.
The term royal fork is sometimes used to describe the situation where the king and queen are forked, and thus being the highest material gaining fork possible. Another term, the grand fork, is sometimes used to describe the situation where the king, queen, and one or both rooks are forked.
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 33...Nf2+ 34.Kg1 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.
Forks are often used as part of a combination which may involve other types of chess tactics as well.
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