A fork is when a piece attacks two other pieces at the same time.
Forks can be particularly effective when
- the attacking piece is of lower value than both the pieces attacked, which includes checking the king
- and also when neither of the attacked pieces is defended.
Forks are good for winning material.
Pins are when an attacking piece is lined up against two opposing pieces and restricts the movement of the first opposing piece because if it moved the second opposing piece would be lost. Pins only work when the second opposing piece is unprotected or of higher value than the attacking piece.
Only bishops, rooks and queens can do the pinning. Pins against the opposing king are particularly effective.
Pins are good for restricting the defending side’s mobility. With the help of another attacking piece they can win material.
In the first example the kinght is pinned against the queen. In the second example it is being attacked by a pawn; either it will be lost or it will move and the queen will be lost. In the last example the white bishop is pinned against the white queen and will be lost.
Skewers are like pins except the more valuable defending piece is the first in line. Skewers only work if either the second defending piece is not protected or is of lower value than the attacking piece. They work best when the attacking piece is of lower value than the first defending piece.
The first defending piece is almost forced to move and the second defending piece is lost.
Only bishops, rooks and queens can skewer things.
Discovered Attacks and Discovered Checks
A discovered attack happens when one piece moves out of the way of another piece, unleashing an attack on an opponent’s piece.
These are sneaky and can be quite deadly, especially if the piece moving out the way can attack something or even take something.
If the opponent’s piece is the king then it is a discovered check. These are often very strong.
In the first example the bishop can move anywhere and unleash a discovered attack from white’s rook on g1 on the black queen. Moving the bishop to b7 or h3 looks like a good idea as it attacks the rook on c8. Black will not be able to save both the queen and the rook.
The second example is a discovered check and white can use it to win the pawn on d6.
In the last example the bishop looks pinned against the white rook. It could move to c2 or e2 defending the rook but it has a much stronger move: Bxh7 giving check. Black has to get out of check and then his queen is lost.
A double check is a special type of discovered check where the piece moving out of the way also gives check.
These always force the opposing king to move – you can’t block a double check or capture a checking piece because the second piece will still be giving check.
They are very good for attacking the opposing king.
In the last example white plays Bf6 double check. Black only has one move: Kd8. But then white plays Rd8 checkmate!