Discovered Attacks and Discovered Check: the piece that leaves strikes

In a discovered attack the piece that moves is not the piece that attacks: the attacker is the one standing behind it. A rook, a bishop or the queen is aimed at a target, and one of your own pieces masks the line. The moment that piece steps aside, the line opens and the attack is already there. The piece that left is then free to do anything on the way out: two attacks in one move, and nobody answers two at once.

Why a discovered attack wins material

The move is 1.Nxd7. The knight leaves the diagonal, eats a rook on the way out, and by leaving it opens the b2-h8 line for the bishop to hit the queen. Black has one move to solve two problems, and no such move exists: saving the queen costs the rook, ignoring the bishop costs the queen. Note that the knight on d7 is safe: neither the king on g8 nor the queen on h8 reaches that square.

Compare it with a fork. A fork asks one piece to find a square that hits both targets at once, and such squares are rare. A discovery splits the labour: the rear piece handles one target, the front piece picks the other, and the two choices are independent. The front piece can capture, check, promote or threaten mate, and the attack behind it happens either way.

The setup: three things on one line

A discovery is not luck, it is geometry, and the geometry always has the same shape. Look for these ingredients before you calculate anything:

• One of your line pieces: rook, bishop or queen. It delivers the blow without moving an inch.
• One of your pieces in between: the mask. The more noise it makes on its way out, the less time the defender has.
• An enemy target at the far end: the king, the queen, a loose rook. File, rank or diagonal, it makes no difference.
• A good square for the front piece: it has to leave with a capture, a check or a threat. Leaving quietly throws the pattern away.

Discovered check takes away the choice

When the target at the far end is the king, the discovery becomes a discovered check, and that changes everything. Against an ordinary attack the defender can ignore you and create a bigger threat. Against check he cannot: the rules force him to answer it on that move, so whatever the front piece did, it did for free. The best known trap in the Petroff Defence shows this in five moves: 1.e4 e5 2.Nf3 Nf6 3.Nxe5 Nxe4 (3...d6 is the correct move) 4.Qe2, and Black retreats with 4...Nf6.

In comes 5.Nc6+. The knight vacates the e-file, the queen on e2 checks the king on e8, and the same knight attacks the queen on d8. A knight and two black pawns attack c6, and 5...bxc6 is illegal: capturing does nothing about the check. The king has no square either, so three moves exist on the whole board and all three block the e-file. 5...Be7 and 5...Ne4 lose to 6.Nxd8; 5...Qe7 loses to 6.Nxe7.

Double check: only the king may move

There is a stronger version still. If the piece that steps aside also gives check, two pieces attack the king at the same time, and that is double check. Against a single check there are three answers: capture the checking piece, block the line, or move the king. Against a double check the first two die: each solves half the problem. That leaves moving the king.

1.Nf6+ is a double check: the knight attacks g8 and, by leaving the g-file, releases the rook. In the same move it attacks the queen on d7. Black has two legal moves on the entire board, and neither defends the queen. 1...Kh8 loses at once to 2.Rg8#, because the knight on f6 covers g8 and the pawn on h7 seals the escape. That leaves 1...Kf8, when 2.Nxd7+ takes the queen with check.

Réti against Tartakower, Vienna 1910

The classic demonstration lasts eleven moves. White hands over the queen on d8 to drag the black king onto the file where the rook is waiting.

10.Bg5+ is the heart of it: the bishop leaves the d-file, the rook on d1 appears, and the bishop on g5 checks as well, all at once. Black owns a queen on e5 that can take on g5 and a knight on e4 that can block on d6, and both ideas are illegal. Two king moves remain and both lose: 10...Kc7 11.Bd8# and 10...Ke8 11.Rd8#.

How to spot them and how to defend

Attacking, the trigger is visual. Defending, it is a question you ask every move.

• Look for your own pieces getting in the way. Your knight in front of your rook is not a flaw, it is raw material.
• Sweep the file, the rank and the diagonals through the enemy king, even when blocked. A block is temporary.
• Piece under attack? Think discovery before you retreat. If it must move, make it open a line and threaten something.
• Be suspicious of an enemy piece that looks hanging. A knight planted on a defended square is usually masking a line.
• Never leave king and queen on one line with an enemy rook or bishop. The pieces in between will move.

The cheapest defence is the usual one: leave nothing loose, and keep the king off files and diagonals that could open. With no target at the far end, the front piece can step aside all day.