Tactics
Discovered Attacks and Double Check
A discovered attack happens when one piece moves out of the way and reveals an attack by a piece standing behind it. The piece that moves is free to make its own threat, so a single move produces two attacks at once. That is why discovered attacks are among the most powerful tactics in chess: the opponent sees one threat coming from a piece that did not even move.
The basic mechanism
Every discovered attack needs two pieces of the same color on one line. The rear piece is a long-range piece: a bishop, rook or queen. The front piece, which can be anything including a pawn, blocks that line. When the front piece moves away, the rear piece suddenly attacks whatever is at the other end of the line. Coaches often call this arrangement a battery or a loaded gun.
The front piece should not move quietly. The ideal discovered attack has the moving piece create a second threat: capturing something, attacking the queen, or threatening mate. Then the opponent faces two problems with only one move to solve them.
Discovered check
When the hidden piece attacks the king, the tactic is called a discovered check. This is where the idea becomes truly dangerous, because the opponent must respond to the check. The piece that moved can go almost anywhere, even to a square where it can be captured, because the opponent has no time to take it.
In the diagram, White's knight on e4 stands between the rook on e1 and the black king on e8. White plays 1.Nc5+. The rook now gives check along the open e-file, while the knight attacks the queen on a6. Black has four legal replies: 1...Kd8 and 1...Kf8 allow 2.Nxa6, while blocking with 1...Qe6 or 1...Qe2 simply hands the queen to the knight or the rook. In every line the queen is lost.
An opening trap with discovered check
The Petroff Defense contains a well-known trap that runs on exactly this idea. After 1.e4 e5 2.Nf3 Nf6 3.Nxe5 Nxe4?, the correct move is 3...d6 first, kicking the knight before taking back the pawn. White now plays 4.Qe2, lining the queen up with Black's king on the open e-file behind the knight on e5. If Black carelessly retreats with 4...Nf6??, then 5.Nc6+ uncovers check from the queen and at the same time attacks the black queen on d8. Black must block the check with 5...Qe7 or 5...Be7, and White collects the queen with 6.Nxd8.
Play through the Petroff discovered-check trap →
The usual answer to 4.Qe2 is 4...Qe7, which puts a piece on the e-file so that no discovered check is possible. That costs Black some time, but it keeps the queen.
Double check
A double check is a discovered check in which the moving piece also gives check. The king is now attacked by two pieces at once. Normally you can answer a check in three ways: move the king, capture the checking piece, or block the line. Against a double check, capturing or blocking can only deal with one attacker, so the only legal answer is a king move. If the king has no safe square, it is checkmate, even if every attacking piece could otherwise be captured.
In this position, 1.Nd6 gives check from the knight and discovers check from the rook on e1. Black's bishop on f8 could capture the knight on d6, and the queen could block on e7, but neither move answers both checks. The king itself is boxed in by its own queen on d8, bishop on f8 and pawns on d7 and f7, and the e7 square is covered by the rook. It is checkmate.
Many short games finish with a double check because the defending king simply has nowhere to go. When you see a double check available, always look at the king's escape squares first. Even if it is not mate, the king is forced to move, and that often lets you follow up with another check or pick up material with the piece that just moved.
The windmill
The windmill (sometimes called the see-saw) is a repeating discovered check. A rook and a bishop work together near the enemy king. The rook gives check, then steps away to capture something with discovered check from the bishop, then steps back to give check again, then captures again. Each time the king must move back and forth between two squares, and the rook eats a piece or pawn on every second move.
The most celebrated windmill occurred in the game Carlos Torre against Emanuel Lasker at Moscow in 1925. Torre sacrificed his queen and then used a rook and bishop pair to give a series of discovered checks, capturing several black pieces and pawns before the defender could escape. Windmills require a specific geometry, usually the enemy king on g8 or g7 with a bishop on the long diagonal and a rook on the seventh rank, so they are rare. When the pieces line up, though, a windmill can win an enormous amount of material.
How to find discovered attacks
- Look for your own batteries. Scan every line your bishops, rooks and queen control. If one of your own pieces is blocking that line, and an enemy king, queen or loose piece sits at the far end, you have a loaded gun.
- Ask where the front piece can go with a threat. The best discovered attack is one where the moving piece makes a second threat: a capture, a check, or an attack on the queen.
- Prefer checks. If the hidden attack is a check, the moving piece can take enormous liberties. Look for squares where it captures something or threatens mate.
- Look for the double check. If the moving piece can also give check, the opponent must move the king. Check the escape squares; you may have mate.
How to defend
Defensively, the warning sign is an enemy long-range piece aimed at your king or queen with only one enemy piece in between. In the Petroff trap above, the danger appeared the moment White's queen arrived on e2 behind the knight. Break the line before it fires: put a piece of your own in the way, move your king or queen off the line, or exchange the rear piece. Never leave your queen on a square that the front piece could attack with a discovered check, because that is precisely where the double threat lands.
How to practise this
- Replay the Petroff trap on our analysis board and find the correct defense for Black at move four.
- Set up the double-check diagram and try moving the knight to other squares. Notice how much weaker the move becomes when the knight does not give check as well.
- Solve themed puzzles regularly on the puzzles page; discovered attacks and double checks appear often among them.
- Read our guides to pins and skewers and forks and double attacks, since all three tactics grow from the same habit of looking along lines.