Discovered attack and double check

A game on lichess.org, 2014, move fifteen. The black queen stands in the centre, on d5, and fears nothing: no white piece attacks it. The white queen stands on d1, on the same file, but between the queens is the white bishop on d3. While the bishop stays put, the queen on d1 is looking at the back of its own bishop.

Once the bishop moves, the file opens. If the bishop goes just anywhere, Black simply moves the queen away. But if the bishop moves with check, Black has to deal with the king, and the queen on d5 stays under attack. Find White's move.

Neither the bishop nor the queen attacked the black queen while they stood still. The attack appeared when one white piece moved out of the way of another. This is the attack an opponent sees worst of all: they watch the piece that moves and miss the one standing behind it that never moved.

What a discovered attack is

In chapter 2 you saw a battery: two of your long-range pieces on one line, one behind the other. Here the set-up is different. In front stands any of your pieces, even a knight or a pawn, and behind it a rook, bishop or queen, with the front piece blocking the back piece's line. The front piece is like a curtain over a window. Pull it aside, and the back piece sees everything on the line.

A discovered attack is an attack by a long-range piece that opens because one of your own pieces has moved off its line. The strength of the idea is that you get two moves in one. The back piece attacks, though it never moved. The front piece, at the same time, does something of its own: gives check, captures a piece, attacks another target. The opponent answers with one move and has no time to meet both threats.

It is a relative of the fork from chapter 11: again two threats with one move. Only in a fork one piece makes both threats, while here two different pieces make them, and the second one attacks from a distance.

The front piece can be any piece: a knight, a pawn, a bishop, a rook, a queen, even the king. The king rarely steps off the line of its own rook or bishop, but in the endgame it happens, and then the opponent's king is checked by the piece that stood behind it. The back piece, though, can only be a rook, bishop or queen: the blow has to travel along a line through the vacated square, and knights and pawns do not strike along lines. So first of all, look at the lines of your long-range pieces and at what is blocking them.

Some batteries turn up so often that they are worth knowing by sight. A bishop on d3 with the queen on c2 behind it, on the b1–h7 diagonal aimed at the castled king. The same bishop on d3 in front of the queen on d1, on the d-file, as in the first puzzle of this chapter. A rook on an open file behind its own knight or bishop. A bishop placed on g2 or b2 behind a knight standing in the centre on the long diagonal. In each of these set-ups the front piece is waiting for a move after which the back piece hits the king, the queen or an undefended piece.

The moving piece gives check

In the first puzzle of the chapter the front piece gave check and the back piece took the queen. This is the most common kind of discovered attack. The check gains time: while the opponent moves the king, the back piece takes what it was aiming at. The front piece may even be lost: the bishop on h7 was lost, but the queen was taken for it.

Count this deal in the piece values from chapter 8. White gave a bishop, three points, and took a queen, nine. But count to the end: had the black queen on d5 been defended, White would have given the bishop and their own queen for the black one and ended up a bishop down. A check with the front piece is like small change: you pay with it so that the back piece can make its move undisturbed, and it is worth paying only if that move is worth it.

Here is how to look for such a blow. Find a long-range piece of yours whose line is blocked by another piece of yours. Look at what stands at the end of that line: an undefended enemy piece, the enemy queen. Then check whether the front piece can move away with check or with a capture. If it can, the blow is ready. The next position shows the same trick, only the line runs along a rank.

Black uses the same trick, and the same sacrifice on h2 is no worse than the one on h7.

The checking piece does not have to sacrifice itself. Sometimes it steps to a safe square, and then the opponent loses a piece and gets nothing in return. Sometimes the back piece also takes its prize with check.

Discovered check

It can also be the other way round: the back piece gives check and the front piece is free. This is discovered check. Chapter 5 mentioned it; now let us see why it is so strong.

After a discovered check the opponent has to protect the king from the back piece. Meanwhile the front piece goes where it likes and does what it likes: takes the queen, attacks a rook, stands en prise. The opponent has no time to take it, because the check must be dealt with first. That is why people say a discovered check is an extra move: the front piece moves twice in a row, and the opponent once.

Hence the rule: when you have a discovered check ready, do not hurry. Pick the strongest square on the board for the front piece: a capture of the queen, an attack on two pieces, another check. Giving a discovered check "anywhere" throws the extra move away.

In chapter 10 you already played a discovered check before you knew its name. In the trap from Petrov's Defence the white knight went from e5 to c6: the queen on e2 gave check along the e-file, and the knight attacked the black queen on d8. Any knight move gave check, but only from c6 did it also hit the queen. That is exactly choosing the best square for the front piece.

The most famous example is the windmill. In Moscow in 1925 the Mexican master Carlos Torre played the former world champion Emanuel Lasker. He gave up his queen, put a bishop on f6, facing the black king, and then his rook went back and forth: giving check from g7 itself, and discovered check from the bishop whenever it left g7. With every discovered check it took something on the seventh rank. Having collected the pawns, the rook won back the queen too, and White won the game. A windmill is a discovered check repeated as often as the opponent cannot get rid of it.

The front piece can be a pawn too. A pawn moves slowly, but it attacks two pieces at once, and if its move uncovers a check, the opponent has no time to save either of them.

Double check

The strongest discovered check is one where the front piece gives check as it moves. This is the double check from chapter 5. You cannot block two checks with one move or take both pieces, so there is only one answer: a king move. If the king has nowhere to go, a double check is mate, even when both checking pieces are en prise.

Before a double check, count not the opponent's pieces but the squares around their king. Which of them are occupied by their own pieces? Which do your pieces strike, including both checking pieces? If there are no free squares, it is mate. If there is one, see whether your next move can take it away too. That is how the mating puzzles below are built: the double check leaves the king one square, and the next move takes it away.

A double check forgives a lot. The front piece may stand where a pawn, the queen or even the king attacks it: it still cannot be taken, because the other piece goes on giving check. That is why the queen is sacrificed for the sake of a double check more often than for anything else. You wrote down the game Réti–Tartakower, in which White gave up the queen and then drove the king into mate with a double check by bishop and rook, in chapter 7.

Which of the two double checks is better? The one after which the knight attacks something as well. Work it out yourself.

A double check most often ends in mate when the king stands at the edge of the board or is hemmed in by its own pieces. Here are three such games.

How not to fall for it

The danger of a discovered attack is that the threat is invisible. The piece that will strike has been standing in place for many moves, and the eye gets used to it. So look not only at what the opponent has just done but at their long-range pieces: rooks, bishops, queen. Which line does each of them stand on? What is further along that line, behind the first piece? If your king or queen stands behind one of the opponent's pieces, that is a loaded battery, and it can fire on any move.

Here is a typical mistake: the queen comes out to the centre and lands right on the line where a battery is waiting for it.

White plays Bxh7+: the bishop takes the pawn with check, the d-file opens, and after the king's reply Qxd5. Exactly what happened in the first puzzle of the chapter. The queen on d5 was not under attack when Black put it there, and that is what fooled them. Before every queen move to the centre it pays to ask: which line will it stand on, and is there an enemy long-range piece on that line, screened by one of its own? If there is, treat the queen as already attacked.

What can you do when a battery is aimed at you? The simplest thing is to take its target off the line: move the queen, step aside with the king, defend the piece. You can put another piece of yours on the line, and then uncovering it achieves nothing. You can attack the front piece of the battery or trade it off before it moves by itself. And it always helps to ask whether the front piece has a move with check: if it has none, the discovered attack is less dangerous, because you will have time to reply.

Sometimes knowledge from chapter 12 helps. The front piece of a battery may turn out to be pinned to its own king or queen. Then it cannot open the line, or can only at a heavy price. Pinning the front piece is one of the best ways to disarm a battery: the opponent is counting on their shot, and their gun is not loaded.

What's next

The fork, the pin, the skewer, the discovered attack: four ways to create two threats at once or catch a piece that has nowhere to go. But often none of them works on its own: a defender is in the way. It stands in place, covers an important square, and while it is there, the blow does not land. Then the defender has to be lured away, or, the other way round, an enemy piece has to be lured onto the square you need. That is deflection and decoy, the subject of the next chapter.