In chess, a fork is a tactic whereby a single piece makes two or more direct attacks simultaneously. Most commonly two pieces are threatened, which is also sometimes called a double attack. The attacker usually aims to gain material by capturing one of the opponent's pieces. The defender often finds it difficult to counter two or more threats in a single move. The attacking piece is called the forking piece; the pieces attacked are said to be forked. A piece that is defended can still said to be forked if the forking piece has a lower value.
Besides attacking pieces, a target of a fork can be a direct mating threat (for example, attacking an unprotected knight while simultaneously setting up a battery of queen and bishop to threaten mate). Or a target can be an implied threat (for example, a knight may attack an unprotected piece while simultaneously threaten to fork queen and rook).[further explanation needed]
Forks are often used as part of a combination which may involve other types of chess tactics as well.
|This article uses algebraic notation to describe chess moves.|
The type of fork is named after the type of forking piece. For example, a knight fork is a knight move that attacks two or more opponent's pieces simultaneously. Any type of piece can perform a fork[note 1]—including the king—and any type of piece can be forked. A fork is most effective when it is forcing, such as when the king is put in check.
Knights are often used for forks. Their unique L-shaped move means that they can attack any other type of piece, including the powerful queen, without being attacked by their targets.
The queen is also often used to fork, but since the queen is usually more valuable than the pieces it attacks, this typically gains material only when the pieces attacked are undefended or if one is undefended and the opposing king is checked. The possibility of a queen fork is a very real threat when the queen is in the open, as is often the case in endgames. If a player wants to force an exchange of queens, forking the opposing queen and king (or an undefended piece) with a protected queen can be useful.
Example from a game
This example 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 forks the white queen and rook; after the queen moves away, Black will win the exchange.
Example from an opening
In the Two Knights Defense (1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6) after 4. Nc3, Black can eliminate White's e4-pawn immediately with 4... Nxe4! due to the fork trick 5. Nxe4 d5—regaining either the bishop or the knight.
Forks can possibly be escaped. A forked piece such as the queen might check the enemy king, a zwischenzug, giving time to move the second forked piece to safety on the next move.
Other terms 
A fork of the king and queen, the highest material-gaining fork possible, is sometimes called a royal fork. A fork of the opponent's king, queen, and one (or both) rooks is sometimes called a grand fork. A knight fork of the opponent's king, queen, and possibly other pieces is sometimes called a family fork or family check.
- Some sources apply the term fork only when a knight is the attacker, while if another piece is the attacker the tactic is called double attack, etc.
- Tissir vs. Dreev, 2004 Chessgames.com
- Burgess, Graham (2009), The Mammoth Book of Chess (3rd ed.), Running Press, ISBN 978-0-7624-3726-9
- Golombek, Harry (1977), Golombek's Encyclopedia of Chess, Crown Publishing, ISBN 0-517-53146-1
- Hooper, David; Whyld, Kenneth (1992), The Oxford Companion to Chess (2nd ed.), Oxford University Press, ISBN 0-19-866164-9