If two Black pieces sit on the sixth rank with three squares between them, they are vulnerable to attack from a White rook that can reach the sixth rank. These two enemy pieces can be attacked in a manner we call a pin, a skewer, or even a fork. The rook can attack both because they are on the same line. These attacks are the relationship I created in the first three exercises for my Essential Tactics worksheets.
Such tactical ideas as pins, skewers, and discoveries take place along a single line—rank, file, or diagonal. A fork also can take place along such a line, but forks are more commonly understood to take place along multiple lines. In this exercise from Essential Tactics, for instance, the correct move places the White king in offensive contact with the Black knight along a diagonal and with the Black bishop along a file.
White to move
Consider this game from the forgotten book, A Treatise on the Game of Chess (1808) by J. H. Sarratt (12-15). The notes are by Sarratt, and the book may be found at Google Books. At three critical moments in this illustrative game, White employs a fork. In the third instance, the fork attacks a king and a target square that must be occupied to put a stop to Black's counterplay.
1.e4 e5 2.Bc4 Bc5 3.c3 Nf6
This move, though very generally played, even by good players, is certainly a bad move.
4.d4 exd4
If 4...Bd6 White would win a piece, as will be shown in a Back-game.
5.e5 Ne4
Instead of this move the Black might play the Queen to his King's second square; or his Queen's Pawn two moves; but you would nevertheless have the best of the game, as will be demonstrated in two Back-games beginning at the fifth move of the Black.
5...Qe7;
5...d5
White to move
6.Bxf7+
Instead of this move, you might play your King's Bishop to the adversary's Queen's fourth square; but he would then sacrifice his knight for three Pawns, as will be shown in the second book.
6...Kxf7 7.Qf3+ Nf6 8.exf6 Qxf6
White to move
9.Qh5+
It seems that if you were to give check at the adv. Queen's fourth square, you would likewise win the Bishop; but he would cover the check with his Queen at his king's third square, and giving you check at the same time, you would be compelled to exchange queens.
9.Qd5+ Qe6+ 10.Qxe6+
9...Qg6 10.Qxc5 Qxg2
White to move
11.Qf5+ Ke8 12.Qf3+- and you will win the game, whether the Black exchange Queens or not.
Modern Grandmaster Play
On the way to winning the US Championship, which concluded Sunday, Hikaru Nakamura had this position with the Black pieces against Ray Robson.
Black to move
29...Qc4
Nakamura forks both rooks, although they easily protect each other. Nonetheless, this move brings the queen to a better square for the assault against the White king.
30.Ref6 a3 31.bxa3 bxa3 32.Qxa3
Black to move
Nakamura must have had this position in mind when he played 29...Qc4. White's move 29 that led to a simple fork of rook was to capture a bishop on e5. Nakumura now initiates a combination that recovers the lost minor piece with interest.
32...Rxc1+ 33.Rxc1 Qxe4+
Black is currently down a rook, but White's moves are forced.
34.Kb2 Qe5+
Another fork.
White to move
35.Rc3 Rb8+ 36.Kc2 Qxf6
With an extra pawn and a less vulnerable king, Black had the better endgame. He used a series of checks to win another pawn and force the rooks off the board.
*See Yuri Averbakh, Chess Tactics for Advanced Players (1992) for definitions of contacts as the foundation of chess tactics.
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