The skewer and the x-ray

A game on lichess.org, 2014, a rook endgame. White has a king on h5, a rook on g5 and a pawn on h3; Black has a king on f4 and a rook on a4. The rooks are far apart, and there seems to be nothing to attack. But the black king and the black rook stand on the same rank, the fourth, with nothing between them.

Put the white rook on that rank and it gives check. The king has to move, and when it does, the rank opens all the way to the rook on a4. Find White's move.

The last chapter promised a pin turned inside out. Here it is. In a pin the valuable piece hid behind, and the piece in front of it could not move. Here the valuable piece stands in front. It can and must move, but as it moves, it exposes the one behind it.

What a skewer is

A skewer is an attack by a rook, bishop or queen on a valuable piece with another enemy piece behind it on the same line. In front stands the king, the queen or a rook. Behind stands something less valuable that is still worth having. The valuable piece may not or cannot afford to stay under attack, so it moves, and the attack goes straight through to the piece behind.

The parties are the same as in a pin: the attacking piece, the front piece and the back piece, all on one line. Only the order changes. In a pin the less valuable piece stands in front, and it does not pay to move it. In a skewer the more valuable piece stands in front, and it does not pay to leave it there. So the check you got into the habit of making in the last chapter is exactly the same: what stands behind a piece on the line of an enemy rook, bishop or queen? If the king stands behind the piece, the piece will not move, and you can attack it. If the king stands in front with a rook behind it, the king will move and expose the rook.

As with the pin, only long-range pieces skewer: the blow has to travel along the line through the square that has been vacated. A knight or a pawn cannot do that. The queen strikes along both straight lines and diagonals, so it has more skewers than any other piece. A rook skewers along ranks and files, a bishop along diagonals.

The English name, skewer, is the image of a kebab: the attacking piece threads two enemy pieces onto one line. The Russian name, "the through blow", says the same thing: the attack passes through the first piece to the second. The word x-ray, which we come to at the end of the chapter, comes from the same picture: a piece sees through another, like an X-ray beam.

Check straight through

The strongest skewer comes with check. A check has to be answered at once, and the opponent has no move left to save the back piece: to move it, defend it or block the line somewhere else. There are three answers to a check, as you know from chapter 5: move the king, block, or take the checking piece. If there is nothing to block with and the checker cannot be taken, the king moves, and the back piece is lost.

So the habit here is this. When the opponent's king stands on one line with one of their undefended pieces and there is nothing between them, look for a square on that line where you can arrive with check. Do it a little further from the king, so that it cannot take the attacking piece, or where one of your pieces defends it. Below are four positions from games: two checks along a rank, by a rook and by a queen, and two checks by a bishop along a diagonal.

Where do you look for such lines? The back rank, when the king has not castled or has come out from behind its pawns. The seventh and second ranks, where rooks arrive in the endgame. The long diagonals, once the pawns have left them. And any line the king has walked onto by itself, running from checks: a fleeing king often lands on one line with one of its own pieces, and the next check goes straight through.

The back piece can be worth more than the attacker. Then the skewer wins the most of all: a bishop that checks the king and takes the queen behind it.

In all four positions the check came from a piece that could not be taken: it stood far from the king or was defended. And in all four the back piece was undefended. Those are the two conditions, and you have to check both before you give the check. What happens when the second one fails we look at below.

Through the queen

The front piece does not have to be the king. The queen cannot stand under attack from a bishop or a rook either: it is worth more than either of them. If a rook or an undefended minor piece stands behind the queen on the line, an attack on the queen wins it.

There is one difference from a check. The rules do not forbid the queen to stay under attack, and the opponent has more answers. The queen can take the attacking piece if it is undefended. It can move away so that it defends the back piece at the same time. Or it can step between the attacker and the back piece and stay on the line. So a skewer through the queen needs one move more of calculation: where will the queen go, and what will it defend from there?

And one more condition. If the front piece is defended, the attacker has to be cheaper than it. A queen that attacks a defended rook achieves nothing: the rook can stay where it is, because taking it means giving the queen for a rook. A rook that attacks a queen does achieve something, even if the queen is defended: standing under attack does not pay for the queen anyway. The more valuable the front piece, the less choice it has: the king must move, and the queen almost always does.

Sometimes there is not one piece behind the queen but a whole chain. Then the attack goes all the way through to the one that cannot be defended.

In the endgame

Most skewers happen in the endgame. There are few pieces, the lines are open, the kings have come out of hiding, and the rooks and queens strike across the whole board. In the endgame the king is an active piece and walks through the centre, so it often ends up on the same line as its own rook. Hence a simple rule for endings: do not put your king on the same rank, file or diagonal as your undefended rook with nothing between them when the opponent has a rook or a queen.

In a rook endgame a skewer often saves a game that looks lost. Here is Black: the white pawn is one step from queening, and an ordinary check will not stop it.

The same trick works the other way round. When you have a pawn on the seventh rank and your rook stands in front of it, on the promotion square, the rook has to get out of the way. If it does so with profit, a skewer wins the opponent's rook that guards the pawn from behind. Such endings are covered in detail in the chapter on rook endings; here we look only at the blow itself.

Skewers also happen often when a pawn promotes. When both sides are pushing pawns to queen, the one who queens first does not always win. A new queen appears on the edge of the board and strikes along long lines. If the opponent's king and queen stand on one of them, the new queen's very first move is a skewer.

So in a pawn race, count not only who reaches the last rank first, but also where the kings and the new queens will stand. If your king stands on one diagonal or rank with the square where the opponent will queen, and your future queen is on the same line, you risk losing it the moment it is born. Experienced players in such endings take the king off dangerous lines in advance, even if it costs a tempo.

When the skewer fails

A skewer looks spectacular, and so it is easy to play one where it gains nothing. Check three things.

1. The back piece is undefended or worth more than the attacker. If a defended knight stands behind the king and the queen gives the check, taking the knight means giving the queen for a knight. The check has won nothing.
2. The front piece cannot defend the back one as it moves. If the king stands next to its rook, it will step to a square next to the rook and defend it. Then the capture is just a trade.
3. The attacking piece cannot be taken, and there is nothing to block with. This is the first thing the opponent looks for, and you have already checked it in the puzzles above.

These two questions also show how to defend against a skewer. Keep your king near your pieces, or at least off an open line it shares with an undefended rook. Defend pieces that stand on one line with your king or queen. And in the endgame, when the opponent's rook roams the whole board, check before every king move: is the king stepping onto a square from which the next check goes straight through?

The x-ray

A long-range piece strikes as far as the first piece in its path. But it keeps in mind what stands behind that piece too: as soon as the front piece moves or is taken, the line opens. This action through a piece is called an x-ray. A skewer is a special case: a blow through the king or the queen that shows itself once they move.

The x-ray turns up most often in exchanges on an open file. Two of the opponent's rooks stand one behind the other, you take the front one, the back one takes back, and it turns out that your second rook was looking at that square through the opponent's piece. The count of attackers and defenders that you have kept since chapter 2 has to include such hidden attacks as well. Look at two positions.

In both positions the x-ray was hidden behind an enemy piece. A defender that stands in front of your rook on the same line does not yet make the square defended: as soon as it moves, your rook comes to life. Check this in every exchange on an open file. Count not only the pieces that attack the square now, but also those standing behind them that will join in after the first capture.

An x-ray can also run through your own piece. From chapter 2 you know the battery and doubled rooks: the back piece defends the front one and looks through it further up the line. If the front rook takes something on that file and is taken, the back one takes back. This is how rooks and the queen line up on an open file before they invade the opponent's camp along it. We come to that in the chapter on open files.

What's next

In the pin, in the skewer and in the x-ray, one piece stands in front of another on a line, and the blow waits for the front piece to move. So far the front piece has been the opponent's. What if it is yours? You move it off the line yourself, the long-range piece behind it gets free play, and meanwhile the piece that moved does something of its own: attacks, captures, gives check. Two threats with one move, and one of them comes as a surprise. That is the discovered attack, and when both pieces give check at once, it is double check. They are the subject of the next chapter.