skewer / X-ray

[phdevos]

Can somebody explain me what the difference between an X-ray and a skewer is exactly? According to the tag descriptions, "An X-Ray attack occurs when one piece attacks a square or piece through another piece. Note that this is not the same as a skewer as the relative value of the piece being attacked through is irrelevant."

I am not sure that the second sentence describes the difference very accurately. Looking at the example, the main difference seems to be that the second piece (the bishop) is attacked twice (once by the rook, and once with a skewer by the queen). You might even argue that it is not so much an attack, but rather a defense, because after the capture, the white queen is defending the rook through the black queen (as opposed to a skewer, where only one attacking piece is involved).

I am asking this, because I am working on the Dutch translation. In Dutch, a skewer is called a "röntgenaanval", which means x-ray attack. What is called an x-ray attack on Chess Tempo, is usually not seen as a separate tactical theme (i.e. it doesn't exist yet :-) )

So I decided to call a skewer "röntgenaanval", and an x-ray attack simply "x-ray". But if I translate the description of an x-ray attack literally, I am not sure that people will understand the difference between the two, as it will be a new tactical concept for the target group. If somebody can come up with a better explanation, it would be most helpful!

[drahacikfm]

My personal definition of X-ray attack is that it always involves two White pieces (assuming White is the one doing the attacking).  The two White pieces are attacking a piece or square in Black's camp: one White piece is attacking directly, and the other White piece is attacking through another Black piece.

On the other hand, a skewer involves only one White piece.

[udo77]

I'm not sure as I'm still a beginner, but that is my understanding:

Look at the example for x-ray: after the first (1...Kh8) move the black king is between the white queen and black rook.

In the skewer example after 2.Qf8+ the white queen is outside the black king and queen.

I admit you can get the wrong idea in the x-ray example because at first it looks like a skewer (it is a skewer in that moment). But the idea is 2.Rxb7 that it becomes an x-ray attack: the white queen attacks/defends b7 through the white queen. Kind of an interference but with the advantage for the other side.

In short the order of the pieces in a line:
skewer or pin: white - black -black
x-ray: black - white - black
(or the colors the other way around).

[phdevos]

Thanks to both! Drahacikfm's explanation is very clear, although somewhat different from the one on the website. I think I should rather use this one instead, since I am not sure otherwise that Dutch users would have the same understanding of the tag as English speaking users. Too bad there's no Dutch word for it...

[marvellosity]

Yes, X-Ray is when you're attacking a piece or square along the same line of action (file, rank, diagonal) as the opponent is defending it. The opponent's piece in between most likely isn't threatened and is entirely incidental.



White checkmates with 1.Rxf8+ because our queen is x-ray attacking the f8 bishop through Black's queen, which is defending it. Note the queen on f6 is not really under threat at all, the point is is that the White queen's action is working through it.



Black just played 1...Rxe2, and White can safely play 2.Rxe2 because our queen is x-ray defending the rook through the black's queen.

So basically the motif is that you're attacking/defending a square through an opponent's piece that is also attacking the square. You are right - an X-Ray attack is often effectively an X-Ray defence. In the example you give, White is effectively attacking b7 twice even though the queen is not attacking b7 directly - that's why it's x-ray.

[udo77]

Yes, X-Ray is when you're attacking a piece or square along the same line of action (file, rank, diagonal) as the opponent is defending it.

... like stabbing in the back. Should be called "back-stabbing" attack