Fork and Double Attack: how to win material in one move

A fork is one move that attacks two things at once. Your opponent saves one and you take the other, because nobody gets two moves in a row. It is the most common tactic in chess and the first one worth mastering. Double attack is the general name; fork is a double attack delivered by a single piece.

Why a fork wins material

The arithmetic is about time. Each move solves one problem; if a single move creates two problems, your opponent solves one and you cash in the other. After 1.Nf7+ the king is in check, and a check cannot be ignored. Black has to play 1...Kg8, and 2.Nxd8 collects the queen. It would not matter if the queen were defended: a knight for a queen is still a fine trade.

That is why a fork with check is the strongest kind. When one of the targets is the king, your opponent loses the right to choose. Without check the fork still works, but the defender sometimes escapes: a bigger counter-threat, one move that defends both, or a capture of the forker.

The knight is the specialist

Every piece forks, but the knight forks best, and the reason is geometric. A knight on a light square attacks dark squares only, never the square it stands on or any square beside it, so its two targets look far apart and the eye fails to connect them. A knight attack also cannot be blocked: against a rook, a bishop or a queen you can throw a piece in the way, and against a knight there is no line to block.

In the diagram, the knight lands on f7, a light square, and attacks d8 and h8, two dark squares. Keep the rule: the two targets of a knight fork always sit on the same colour, opposite to the knight. Four arrangements cover almost every knight fork you will meet:

• Same rank or file, two squares apart: d7 and f7 fall to a knight on e5.
• Same rank or file, four squares apart: d8 and h8 fall to a knight on f7.
• Diagonally touching, right next to each other: f7 and g6 fall to a knight on e5.
• Same diagonal, three squares apart: f7 and c4 fall to a knight on e5.

Every piece forks

The pawn gives the cheapest fork: it attacks two diagonal squares, and any piece that steps onto them is worth more than it is. The bishop works on two crossing diagonals, so a bishop checking along one often eats something along the other. The rook forks in a cross, classically from the seventh rank. The queen attacks in eight directions and creates more double attacks than any piece, but she gains nothing by taking a defended piece worth less than she is.

The pawn fork worth knowing by heart appears in the opening and has a name: the fork trick. After 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Nc3, Black plays 4...Nxe4 and hands over a knight. White wins it back with 5.Nxe4, and now comes the point.

The usual answer is 6.Bd3, which pulls the bishop out of the attack and defends the knight on e4 into the bargain. After 6...dxe4 7.Bxe4 material is level again: Black has the piece back, a solid pawn on e5 and a half-open d-file, since White still has the pawn on d2. The sacrifice was never a sacrifice, it was a one-move loan.

How to see a fork before you play it

Tactics do not turn up by luck. They turn up when the position holds the right ingredients, and there are few of them. Before you calculate anything, look for the signals:

• Loose pieces. Any undefended enemy piece is a target already prepared. Two loose at once ask for a move that hits both.
• The king plus anything. The king is always on the board. Whenever you spot a check, ask what else that move attacks.
• Targets on one colour. Two pieces on light squares can fall to a light-squared bishop, to the queen, or to a knight landing on a dark square.
• Landing squares. Walk an imaginary knight around the empty squares near the enemy king and check what each one would hit.
• Pieces bunched together. King, queen and rook in the same corner is an invitation. The closer they stand, the less has to go wrong.

The king steps aside with 1...Kf8 and 2.Qxa8+ takes the rook. The queen used two diagonals crossing on d5, and all three squares are light: a light-squared bishop there would do the same damage. The loose rook is what created the tactic. Give a8 a defender and the check is only a check.

The habit: checks, captures and threats

Strong players do not hunt for forks. They run a fixed scan every move and the forks turn up on their own. Three questions, always in the same order: what checks do I have, what captures, what threats. Then run the same three from the other side, for the move you intend to play. At first it costs a minute a move. Within weeks it costs seconds.

How to stop getting forked

Defending against forks is almost all prevention: once the knight has landed on the right square there is usually nothing left to do.

• Leave no loose pieces. A defended piece can still be forked, but now the fork costs the forker something.
• Watch the colours. A king and queen on opposite colours are safe from the same knight, always.
• Control the landing squares. A knight needs a safe square to arrive on, and h6 or h3 does more than give the king air.
• When in check, look at the whole board before moving the king. Blocking or capturing the attacker sometimes rescues the second piece.
• Before you capture, ask what the captured piece was defending.