Discovered attacks

The queen was not attacked. You checked. Then a knight moved, and a rook you had stopped looking at three moves ago took it.

That is a discovered attack. One piece moves out of a line, and the queen, rook or bishop standing behind it is suddenly attacking something. Usually the piece that moved makes a threat of its own as well, so one move makes two.

Most players look at a board by asking what each piece attacks. A discovery gets past that, because the attacker is not attacking anything yet. It is blocked by a piece of its own, and the blocker is the piece you are watching. The mistake is to count the piece in front. The danger is the piece behind it.

One move, two threats

Count it the way hanging pieces teaches and White is in trouble. The bishop on e4 is attacked twice, by the knight on f6 and the queen on e7, and defended once, by the rook on e1. The honest move looks like a retreat.

Now look at what the bishop is standing in front of. Behind it, straight down the e-file, the rook on e1 is aimed at the black queen. The bishop is the only thing in the way.

1.Bxh7+. The bishop moves, and it moves with check, so Black has to spend the next move on the king: 1...Kxh7, 1...Nxh7 or 1...Kh8. None of them does anything about the e-file, and 2.Rxe7 takes the queen. White has given a bishop for a queen and a pawn.

The rook did not have to move to become dangerous. A move ago it was the bishop's only defender. One bishop move later it was attacking the queen.

Here is the part that matters. Every bishop move in this position uncovers the rook, and only one of them wins. After 1.Bf3 or 1.Bxc6, Black simply moves the queen and the position is level. After 1.Bxh7+, Black has something more urgent to do than move the queen, and does not get the chance. So the piece that steps away needs a threat your opponent has to answer first, and a check is the strongest kind.

Discovered check: the piece that moves is free

Turn it round and let the uncovered piece give the check. Now the piece that moves can do anything it likes.

The bishop on b3 is aimed at g8, and White's own knight on d5 is all that stands in the way. The moment the knight steps off d5, Black is in check, whatever the knight did when it moved.

So it takes the queen. 1.Nxb6+, and the pawn on a7 is guarding b6, and that does not matter. Black is in check and must deal with it first, with 1...Kh8 or by blocking the diagonal (1...Be6, 1...Nd5, 1...Rf7). By the time Black is free to recapture, the knight has left: 1...Kh8 2.Nxa8 and it takes a rook on the way out.

Look at what Black was counting in this position. The knight on d5 is attacked by the pawn on c6 and the knight on f6, and it is defended twice. Black was watching the knight. The bishop behind it was the problem.

A piece that uncovers a check cannot be taken for one move. It can capture a defended piece, stand on an attacked square, or walk into the middle of the enemy army, because nothing gets to capture it until the check has been dealt with. Every knight move here is check, so the question is not whether to discover but what to take. Nxf6+ and Nc7+ also win material. Nxb6+ takes the biggest thing on the board, and the engine puts it well ahead of the rest.

The same thing, pointed at you

Now it is your queen. It sits on e2, Black's rook sits on e8, and the only thing between them is Black's knight on e4. Nothing is attacking your queen. That is exactly how it feels during the game, and that is why it gets lost.

If it were Black's move, 1...Ng3+ would end it. The knight gives check, so you take it (2.hxg3 or 2.fxg3) or move the king, and 2...Rxe2 takes the queen either way.

It is your move, and the natural ones all lose. 1.Nd2 attacks the knight, which feels like the answer and changes nothing, because the knight was about to move anyway. 1.Rfe1 defends the queen, and it goes anyway: you get a rook and a knight for it and are still clearly worse. Taking the knight with 1.Qxe4 loses the queen at once, because the rook and the bishop on b7 both guard e4.

What works is the list from breaking a pin: take the queen off the line, or put something on it. 1.Qd3 does the first. 1.Ne5 does the second, and it is the engine's choice, because the knight blocks the file and attacks the black queen at the same time. Either one keeps your queen.

The tell

A discovered attack is set up a few moves before it happens, and you can see it coming if you look at lines instead of pieces.

1. Find every enemy queen, rook and bishop, and follow its line to the end. Include the lines that are blocked.
2. If the line reaches your king, queen or a rook, see what is standing in the way. If it is your own piece, that is a pin. If it is one of theirs, the gun is loaded.
3. Ask what that piece can do when it moves. If it can move with check, or capture something, act this move. Move the target, block the line, or take the piece in front if it is not defended.

The line people miss is the one blocked by an enemy piece, because a blocked line does not look like an attack. Nothing on it is attacking anything, until something moves.

The habit

When you do your blunder check, look past the pieces. Any enemy piece standing between an enemy queen, rook or bishop and your king or queen counts as already moved. Ask where it could go with check, and whether your queen survives when it does.

Then turn it round. Before every move, look along your own lines for a piece of yours sitting in front of your queen, rook or bishop and pointed at something that matters. That is a discovery you already own. When the engine tags a move as a missed win, this is one of the shapes to look for: the move was there, and the piece that had it looked like it was only blocking.

Then go and see what it has cost you. Filter your games by hanging a rook or queen and look for the losses where your queen was not attacked the move before, and filter by missing a winning move for the discoveries you had and did not play. Look for both.