X-rays, skewers, dogs and cats

[interlist]

This might be better titled "X-rays and Skewers (Slight Return)", as there has been prior discussion on this topic:

   [Oct 16, 2009 -> Oct 16, 2009]   skewer  / X-ray

   [Nov 15, 2009 -> Dec 03, 2009]   Skewer, Pin and X-Ray Attack

   [Sep 09, 2009 -> Jan 03, 2011]   What is a skewer according to you ?

It is with some trepidation that I make this posting, arising from some comments between Alvaro and myself on problem 2682.  

(Spoiler Alert) - Here is the starting position for the problem.



I want to empathically state, for the record, that I have come to understand and appreciate the emphasis many of the players on CT put on correct tagging.  And I don't believe there is any disagreement between Alvaro and myself that the correct tag for this problem is unequivocally "skewer".  

The controversy, such as it may be, is about nomenclature.  That is to say, whether or not my assertion that

all skewers are also X-ray attacks

is correct. I can show a simple diagram of this:

Code:

+--------------------------------+
|         X-Ray Attacks          |
|                                |
|                                |
|                                |
|                                |
|                                |
+--------------+                 |
|   Skewers    |                 |
|              |                 |
|              |                 |
|              |                 |
|              |                 |
|              |                 |
|              |                 |
+--------------+-----------------+
 

Now, I believe it is incorrect to say that skewers are not X-ray attacks, similar to saying that dogs are not mammals.  To say dogs are not cats is correct, but to say dogs are not mammals is not (correct). And whether it is better to refer to a dog as a dog or a mammal depends on context.  Both are correct, but "dog" will typically be preferred. (Sorry to be so pedantic in my rhetoric - but I want to be clear)

Similarly, there is no issue on tagging, since it is always better to utilize more specific labels when applicable. And I think this should be emphasized in the descriptions of skewers and X-ray attacks in the CT Tactical Motif webpage:

Chess Tempo - Tactical Motifs definitions

Skewer
   The player attacks a piece of the opponent, which cannot move
   without exposing a less valuable square or piece behind it to
   attack. The front piece usually moves, allowing the piece behind it
   to be captured.


X-Ray Attack
   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 believe it would be less confusing, instead of saying X-ray Attacks are "not the same as a skewer", to say that skewer is the preferred term since it is more specific, and it should have priority when applicable.  This is similar to most people calling a dog a dog, instead of a mammal (and never a cat!).

--interlist

(Random QotD - "Discretion may be the better part of valor, but sometimes indiscretion is more fun!")

Flickr link: http://www.flickr.com/photos/86304021@N08/

[oded ross]

X-Ray attack, the way it is used here, should have been long ago renamed as X-Ray Defense.
That would have solved many tagging discussions including this one.

[interlist]

An artistic interpretation of X-rays, skewer, and pins The Black bishop on a1 is X-ray attacking the bishop on h8 thru the d4-pawn. The White queen is X-ray attacking the Black king thru the queen on e7, but the more specific term pin applies, since the queen cannot move. (From a real game where the author, playing Black, was soon to lose)

Many discussions on the definitions of various tactical terms refer to Wikipedia pages for their definitions.  These definitions have the merit of being widely accessible, and having the support of collective mass consensus (at least one hopes so).  In conformance with such a tradition I will include brief excerpts of the Wiki definitions here (they of course generally support my thesis that skewers are a variety of X-ray attack):

Skewer
In chess, a skewer (or X-ray attack) is an attack upon two pieces in a line and is similar to a pin. In fact, a skewer is sometimes described as a "reverse pin"; the difference is that in a skewer, the more valuable piece is in front of the piece of lesser or equal value. The opponent is compelled to move the more valuable piece to avoid its capture, thereby exposing the less valuable piece which can then be captured (see chess piece relative value). The long-range pieces (queen, rook, and bishop) can skewer.

X-ray Attack
In chess, the term X-ray or X-ray attack is sometimes used
  (1) as a synonym for skewer. The term is also sometimes used to refer to a tactic where a piece either
  (2) indirectly attacks an enemy piece through another piece or pieces or
  (3) defends a friendly piece through an enemy piece.

Such citations have been done in the other CT threads referenced in my earlier article.  There is a CT-centric argument which gets made that CT definitions should trump other definitions.  Certainly such a the case can (indeed should) get made when there is ambiguity in the other definitions. Or when usage of other definitions conflict.  The CT tagging system needs precision and consensus among its community.  But it still is bound by general conventions, and must respect widely disseminated meanings.  It should strive to conform with outside usage and accepted practice as best as it can.  And it should provide novice users with a haven of consistency allowing the utmost leveraging of their current understanding of the game.

Arguing thus, we see that the overlap of skewer and X-ray attack is acknowledged by the community at large.  And that the term skewer is specific, whereas X-ray is much more generic.  In fact (having just read Oded's post as I write), X-ray is so generic as to mean action-thru-a-piece in essence. And can be used in both the attacking and defending sense.  I agree with Oded that it would be advantageous to distinguish between X-ray attack/defense.  But I started this thread specifically because of Alvaro's categorical statement that problem 9682 was "NOT AN X-RAY", when I would have preferred "SHOULD NOT BE TAGGED AS AN X-RAY".    

Let me for the moment quote extensively from a widely respected historian of the game - Edgar Winter.  Even if one finds the current arguments tedious, his treatment of the search for origins of the terms is less so.  And it might help to understand the usage (and the unfortunate confusion which sometimes arises).  He also comments on how the terms are utilized in other cultures.  I, being American, really only understand the English usage of the phrase. Luckily for me, one learns from Winter's research that the term X-ray attack was originated by an American, Kenneth Harkness, a long-time editor of US Review (and inventor of one of the first rating systems in chess):

First, a brief overview from Chess Explorations (12) by Edward Winter:

   Section One: Earliest Occurrences of Chess Terms

   Skewer
       [Edgar] Pennel's teaching is original in the
       extreme. Bystanders at Blackpool [i.e. at the British Chess
       Federation Congress in 1937] wondered at the strange terms such
       as "skewer" with which the boys interlarded their
       conversation. Explanations revealed that it was a term coined
       by Mr Pennell ... -- CHESS, 14 January 1939, page 212.

   X-Ray attack
      There is another type of double attack in which the targets are
      threatened in one direction. The attacking piece threatens two
      units, one behind the other, on the same rank, file or
      diagonal. This double threat has lacked a good descriptive
      name. We suggest "X-Ray" attack. -- Article by K. Harkness on page
      25 of the Chess Review, April 1947. See C.N. 4245.

 

Next, an extensive exploration of the terms, from Edgar Winter's Chess Notes C.N. 4204-4273 and C.N. 4274-4337:

4231. Skewers

   This position comes from page 26 of Alexander on Chess by
   C.H.O -- D. Alexander (London, 1974): dia01

   Alexander's caption reads:

      Three examples of the skewer. White bishop skewers R through the
      Q and the White rook skewers Kt through the K; Black Q skewers B
      through K at bottom of diagram.'

   C.N. 3061 (see page 102 of Chess Facts and Fables) mentioned that
   Edgar Pennell coined "skewer" in the late 1930s. The term is absent
   from the Small Chess Dictionary by Y. Averbakh (Belgrade, 1980),
   and translations into various languages will therefore be
   welcomed. In German, for example, Spiess is the rough equivalent,
   although it has no entry in German-language chess encyclopaedias
   and dictionaries. As regards Spanish, page 21 of Enciclopedia del
   ajedrez by H. Golombek (Barcelona, 1980) translated skewer by
   ataque "a través".

   Another term for skewer in English (particularly in the United
   States) is x-ray attack. When did that make its first appearance in
   chess literature?

4235. Skewers (C.N. 4231)

   Turning to the French term for skewer, Hassan Roger Sadeghi
   (Lausanne, Switzerland) quotes the definition of "enfilade" on page
   31 of Le guide des échecs by N. Giffard and A. Biénabe (Paris,
   1993):

   Attaque, avec une pièce à longue portée, d'une pièce adverse
   derrière laquelle se situe, sur la même ligne d'action, une autre
   pièce du camp opposé. Si la première fuit, la deuxième se fait
   prendre. C'est tout l'intérêt de cette attaque.'

   The entry for 'enfilade' on pages 131-132 of the Dictionnaire des
   échecs by F. Le Lionnais and E. Maget (Paris, 1967) limited itself
   to cases where check is given, commenting that "certaines études
   artistiques sont fondées sur les échecs enfilade". The example
   presented by the Dictionnaire was a study by Henri Rinck from page
   217 of the July 1903 Deutsche Schachzeitung: dia01

   "1 Ra8 If 1...Qxa8 2 Bf3+ enfilade; if 1...Qh7 2 Bg6 Qxg6 3 Ra6+
   enfilade; if 1...Qc4 2 Rc8+ enfilade; if 1...Qa2 2 Rxa4 and if
   2...Qxa4 3 Be8+ enfilade, etc."

4236. Skewers (C.N.s 4231 & 4235)

   Pages 65-66 of the Diccionario de ajedrez by A. Gude (Madrid, 2005)
   use the term "enfilada", adding that it is also known in Spanish as
   "ataque en línea".

4271. Skewers (C.N.s 4231, 4235, 4236 & 4245)

   Christian Sánchez (Rosario, Argentina) notes that for the skewer
   motif Milton L. Hanauer used the term "hurdle"; see, for instance,
   page 56 of Hanauer's book Chess Made Simple (New York, 1957).

   Our correspondent also mentions that the Spanish translator of
   B.H. Wood's Easy Guide to Chess had difficulty in finding an
   equivalent for "skewer", as explained in a footnote on page 66 of
   Camino fácil del ajedrez (Buenos Aires, 1944):

   "Este tipo de elemento de combinación es una mezcla de 'doble' y
   'clavada', con preferencia lo primero, y su clasificación por
   separado creemos es la primera vez que se efectúa. Hemos adoptado
   el vocablo original inglés, pues es difícil hallar un término tan
   expresivo en el idioma castellano, ya que la traducción literal de
   'skewer' (espetador, o aguja de ensartar) no suena muy bien, y
   hasta que sea propuesto algún vocablo equivalente y práctico, como
   sucede con varias expresiones del diccionario ajedrecístico."

4324. Skewers (C.N.s 4231, 4235, 4236, 4245 & 4271)

   Sandro Litigio (Como, Italy) writes:

   The Italian term for skewer is "infilata". The entry on pages
   278-279 of the Dizionario Enciclopedico degli Scacchi by Adriano
   Chicco and Giorgio Porreca (Milan, 1971) limited itself to cases
   where check is given:

      "È lo scacco al Re, dietro il quale sulla stessa linea di azione
      del pezzo che dà scacco, trovasi un altro pezzo avversario ..."

   The example presented was a study from Civis Bononiae (1454).

      "1 Rh5, and if 1...Rxh5 2 Ra6+ Kc5 3 Ra5+ e con lo scacco di
      infilata il Bianco guadagna la Torre."

* * * * * * * * * * *

4245. X-ray attack (C.N. 4231)

   C.N. 4231 asked when ‘x-ray attack’ first appeared in chess
   literature. Jack O’Keefe (Ann Arbor, MI, USA) writes:

   "The term 'x-ray attack' was apparently originated by Kenneth
   Harkness. On page 25 of the April 1947 Chess Review, in his series
   'Picture Guide to Chess', he mentioned forks and then wrote:

     'There is another type of double attack in which the targets are
      threatened in one direction. The attacking piece threatens two
      units, one behind the other, on the same rank, file or
      diagonal. This double threat has lacked a good descriptive
      name. We suggest "X-Ray" attack.'"

   The article by Harkness (pages 25-30) gave many illustrative
   positions and used the term x-ray countless times.

As I said in the comments, Winters' research is both meticulous and accurate.

--interlist

Flickr link: http://www.flickr.com/photos/86304021@N08/

[alvarofrota]

Rio de Janeiro, 4 de setembro de 2012.

Prezado interlist:

Lets say that Chess Tempo usage of an X-ray attack (or an X-ray defense, what is the same thing) is a restricted one.

In the more widely view you are bringing us, an X-ray attack is a pressure alongside a line (not an attack or a defense in the strict sense of this words) of a piece of a player against a piece (or a King) of the adversary, through another piece of the adversary.

Pressure is rather a strategical concept and attack or defense is rather a tactical one. Thus, in the strict sense of the Chess Tempo definition, you can count an X-ray attack (or and X-ray defense) when counting the attacks and defenses of a piece in order to define if it can be safely took or not.

Despite all the historical arguments, the Chess Tempo strict definition is much more useful to the practical objective of our Tag System: to construct personal sets of problems with a class of tactical motif that can be used to perform some tactical themes.

In the first example above, the X-ray attack is a truly tactical motif that allows White to win material:

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Example nº 1"] [Site "www.chesstempo.com"] [White "White plays"] [Black "And wins material"] [WhiteElo "-"] [BlackElo "-"] [Result "1/2-1/2"] [FEN "2n5/1n6/4k3/N2b4/8/6K1/6B1/8 w - - 0 1"] {The White Bishop is truly attacking the "b7" Black Knight **through** the "d5" Black Bishop. The "d5" Black Bishop **not** interferes with the action of the White Bishop. Even the White Bishop is not capable to capture the Knight in the first move, there is two attacks and only one defense, allowing White to win a piece:} 1. Nxb7 Bxb7 2. Bxb7  1/2-1/2

In the second example, however, there is no suck tactical motif (despite there is a pin tactical motif) and White do not win material:

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Example nº 2"] [Site "www.chesstempo.com"] [White "White plays"] [Black "But not wins material"] [WhiteElo "-"] [BlackElo "-"] [Result "1/2-1/2"] [FEN "2b5/1n6/4k3/N2n4/8/6K1/6B1/8 w - - 0 0"] {The White Bishop is **not** attacking the Black Knight at "b7".  As the "d5" Knight *interferes* with his action, the White Bishop  is NOT capable to capture the "b7" Knight. There is just one attack and one one defense and White is **not** allowed to win a piece:} 1. Nxb7 Bxb7 1/2-1/2

And, last but not the least, in the third example we have an X-ray defense that is not an skewer:

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Exemple nº 3"] [Site "www.chesstempo.com"] [White "White plays"] [Black "And wins"] [WhiteElo "-"] [BlackElo "-"] [Result "1-0"] [FEN "8/8/6R1/5k2/Q7/3K4/8/8 w - - 0 0"] {White mates the Black King} 1. Qe4# {using an **X-ray defense** over his Rook. Here there is **not** an **skewer**.} 1-0

Aquele abraço!

Álvaro Frota

[interlist]

Hi Álvaro,

Thank you for the last post with the examples, I think I better understand your thinking on this matter.  Hopefully this post will help you understand mine.

Rio de Janeiro, 4 de setembro de 2012.

Prezado interlist:

Lets say that Chess Tempo usage of an X-ray attack (or an X-ray defense, what is the same thing) is a restricted one.

This serves to underscore a difference in our thinking.  First of all, the concept of an X-ray attack is not the same as the CT tag X-ray Attack.  I think the discussion below, especially of your second example, will demonstrate this.

I view the tags as being in a hierarchy, with the more specific taking precedence over the more generic.  So I don't think of the CT usage of X-ray as being more restriction, but actually, less restrictive.  An X-ray attack can be involved in both a skewer and a pin for instance (again, see example #2 below).

In the more widely view you are bringing us, an X-ray attack is a pressure alongside a line (not an attack or a defense in the strict sense of this words) of a piece of a player against a piece (or a King) of the adversary, through another piece of the adversary.

Pressure is rather a strategical concept and attack or defense is rather a tactical one. Thus, in the strict sense of the Chess Tempo definition, you can count an X-ray attack (or and X-ray defense) when counting the attacks and defenses of a piece in order to define if it can be safely took or not.

Despite all the historical arguments, the Chess Tempo strict definition is much more useful to the practical objective of our Tag System: to construct personal sets of problems with a class of tactical motif that can be used to perform some tactical themes.

Yes, I completely agree with the first paragraph.  I am indeed asking you to adopt the wider view of the definition of X-ray attack, and to keep the distinction between the generic concept and its usage as a tag.  So, I don't think it correct to think of the CT tag as being strict, but rather more of a catch-all for cases where more specific tags don't apply.  Thus, skewer and pin are stricter.

Now, you make an interesting point about pressure versus attack and defense.  The terminology depends on context of course.  Consider the move 1.e4, it puts pressure on d5.  But is it attacking or defending the square?  Normally we say it attacks d5, since d5 is on the opponent's side of the board. Compare that to the move 6...a6 in the Najdorf where we usually say the pawn defends b5.  Also, if after 1.e4, we manage to get a knight on d5 our terminology would change and we now say the pawn is defending d5.

So then, in a simple X-ray attack combination, the usual usage is to say that the projecting piece is attacking, even if the projecting piece ends up defending after the combination is executed.

The X-ray attack represents potential energy, much like a boulder sitting on top of a hill.  How it gets unleashed, or even if it gets unleashed, depends on the details of the combination.  Let's turn to the examples you provide, and explore the motifs.

In the first example above, the X-ray attack is a truly tactical motif that allows White to win material:

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Example nº 1"] [Site "www.chesstempo.com"] [White "White plays"] [Black "And wins material"] [WhiteElo "-"] [BlackElo "-"] [Result "1/2-1/2"] [FEN "2n5/1n6/4k3/N2b4/8/6K1/6B1/8 w - - 0 1"] {The White Bishop is truly attacking the "b7" Black Knight **through** the "d5" Black Bishop. The "d5" Black Bishop **not** interferes with the action of the White Bishop. Even the White Bishop is not capable to capture the Knight in the first move, there is two attacks and only one defense, allowing White to win a piece:} 1. Nxb7 Bxb7 2. Bxb7  1/2-1/2

OK, I have some reservations about your statements here. An X-ray attack is independent of the blocking piece, and the fact that it's a bishop here and a knight in your second example doesn't alter the fact - the knight on b7 is being X-ray attacked in both cases.  You are confusing your definition by requiring that the X-ray attack be exploitable. The usage of the tag implicitly yields this, since there will be a combination utilizing the X-ray attack for material gain.  But again, a tag is not a definition.  And not every X-ray attack is exploitable.

So, the white bishop is not truly attacking the knight on b7, after 1.Nxb7 Bxg2 2.Kxg2 and the bishop never even gets to b7.  But, yes, White still wins a piece utilizing an X-ray attack.  (Similarly, before you edited your first example Black could have obtained a drawn K+2N vs K game vs. the winning K+B+N vs. K ending - the X-ray attack being short-circuited by BxB).

Again, I caution against conflating the exploitation of a X-ray Attack with its definition.  

In the second example, however, there is no suck tactical motif (despite there is a pin tactical motif) and White do not win material:

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Example nº 2"] [Site "www.chesstempo.com"] [White "White plays"] [Black "But not wins material"] [WhiteElo "-"] [BlackElo "-"] [Result "1/2-1/2"] [FEN "2b5/1n6/4k3/N2n4/8/6K1/6B1/8 w - - 0 0"] {The White Bishop is **not** attacking the Black Knight at "b7".  As the "d5" Knight *interferes* with his action, the White Bishop  is NOT capable to capture the "b7" Knight. There is just one attack and one one defense and White is **not** allowed to win a piece:} 1. Nxb7 Bxb7 1/2-1/2

OK, here we have a great example.  Again, the bishop is still X-ray attacking the knight on b7.  Your assessment of the knight is of course correct, for that position.  But imagine that there is a pawn on c3, then if White plays 1.c4 - the knight either falls to the pawn, or is driven away, thereby unscreening the X-ray attack.  This is a combination unleashing the potential of the X-ray attack, and now you do have a CT-problem with the "X-ray Attack" tag. All you needed was a lowly pawn.

And if instead you put a Black pawn on c3 and a white pawn on d2, White could play dxc3 winning a pawn. The tag would be "Pin", because the N on d5 is pinned by the X-ray attack on b7.  Again, context determines the tag used. But underpinning (pun intended) all three of these positions is the X-ray attack of the bishop - even though only two of them hit paydirt.

Now however, I better understand your comments from problem 9682:

  More than this, in an *X-Ray Attack* the line of action of the
   pieces must be the same. For instance, a Bishop can *Skewer* a Rook
   against an undefended minor piece, but there is not an *X-Ray
   Attack* of a Bishop through a Rook. Also, a Bishop, a Rook or even
   a Queen can *Skewer* a Knight against an undefended Pawn but an
   *X-Ray Attack* through a Knight is something that does not exist.

At first I was very confused, and I still am a little confused (examples would help) - but I think you are describing situations necessary for an X-ray attack to be exploited.  Of course I disagree with these restrictions on what constitutes a X-ray attack.  The definition is very generic, and requires no collinearity between the action of attackers and defenders.
 

And, last but not the least, in the third example we have an X-ray defense that is not an skewer:

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Exemple nº 3"] [Site "www.chesstempo.com"] [White "White plays"] [Black "And wins"] [WhiteElo "-"] [BlackElo "-"] [Result "1-0"] [FEN "8/8/6R1/5k2/Q7/3K4/8/8 w - - 0 0"] {White mates the Black King} 1. Qe4# {using an **X-ray defense** over his Rook. Here there is **not** an **skewer**.} 1-0

This last example is a great one.  'Nuff said.  (Update - just did 58066, thanks Álvaro, very topical)

Álvaro let me finish by stating my respect and admiration of your work here on CT. You show boundless enthusiasm in your role as educator, and I've recognized the excellence of your comments for a long time from my problem solving.  I've even google translated some of your posts in the forum.  It is because of this respect I've taken so much time to make these posts. Anyways, thanks for reading and especially, writing.

Regards and Aquele abraço!

--interlist

[alvarofrota]

Rio de Janeiro, 4 de setembro de 2012.

Prezado interlist:

Thanks for you kind words.

I am just trying to explain to you what is the Chess Tempo definition of an X-ray attack and how it works on the board. Please, see two more examples:

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Problem nº 31790"] [Site "www.chesstempo.com"] [White "White plays"] [Black "And wins"] [WhiteElo "-"] [BlackElo "-"] [Result "1-0"] [FEN "r4k2/4npp1/1P1r4/N1P5/6p1/2P2p2/P1K2P2/R7 w - - 0 1"] {White initiates taking the hanging Rook} 1. cxd6  {and Black exchanges the Knights} Rxa5 2. dxe7+  {winning a Pawn} Kxe7  {Then, White advances his passed Pawn} 3. b7 {and Black tries to attack the Pawn from behind} Rb5 {but White *X-ray defends* the passed Pawn through the line of action of the Black Rook} 4. Rb1  { forcing Black to give up his Rook in order to prevent the queening} Rxb7 5. Rxb7+ {and in the end White is a Rook up.} 1-0

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Problem nº 54798"] [Site "www.chesstempo.com"] [White "Black plays"] [Black "And wins"] [Date ""] [Round ""] [Result "0-1"] [FEN "6k1/1p3ppp/4b3/3p4/q7/N1Q5/1P3PPP/r1R3K1 b - - 1 1"] { After Black took advantage of the *X-ray attack* of his "a1" Rook, through the "c1" White Rook, over the "d1" square,} 1...Qd1+  { White tries to defend hinself retreating his Queen} 2.Qe1 {to a square where she is *X-ray defended*, through the "d1" Black Queen, by his "c1" Rook. However, the X-ray attack of the "a1" Black Rook, through the "c1" White Rook, forms a battery with the "d1" Black Queen, attacking the "e1" White Queen and allowing} Qxe1+ {and now the "a1" Black Rook X-ray defends, throug the "c1" White Rook, the "e1" Black Queen. Thus, after the forced retake} 3.Rxe1 { a back rank mate follows: }  Rxe1#  0-1

Problem number 98318 is a very good example of a Queen *sacrifice* that uses an *X-ray attack* to deliver a checkmate:

( ) vs ( )

Date:

Event:

Site:

Round:

Result:

[Event "Problem nº 98318"] [Site "www.chesstempo.com"] [White "White plays"] [Black "and wins"] [WhiteElo "-"] [BlackElo "-"] [Result "1-0"] [FEN "r3r2k/pp1n3p/5P1B/2Q5/3pb3/3q2P1/PP4BP/R4R1K w - - 1 1"] {After} 1.Bg7+ Kg8  { White takes advantage of the *X-ray attack* that the "g2" White Bishop exercises over "d5" through the "e4" Black Bishop to *sacrifice* his Queen delivering a check } 2.Qd5+  { that evolves into a checkmate: } Bxd5 3.Bxd5+ Re6 {and} 4.Bxe6#  1-0

Aquele abraço!

Álvaro Frota

[interlist]

I wonder if the different language/cultures contribute to some of this - but the concept of X-ray attack is quite clear and widely used among English speakers (see Winter/Wiki/etc).  What might be more complicated is the CT tag "X-ray Attack". The reason for this is because the actual exploitation of the X-ray attack can involve complicated tactical maneuvers.   If the original comments were specifically addressing tagging I might never have commented in turn, but they appeared to redefine the concept of the x-ray attack.

The concept and definition of an X-ray attack is unambiguous if one adheres to Harkness' original definition, which is my main point. 

I've gone back to my previous post, and diagrammed the examples to illustrate the X-ray attack present in each and every one of them.  I cleaned up a little my modifications to example 2 to make it more realistic and coherent in a minor fashion as well.  So before I look at the new examples Álvaro has sent us, let me review once more the earlier (and simplier) examples.

CT Problem 2682. After 1...Qb7 Black's queen skewers White's king who must move, unscreening the X-ray attack (shown in red) on h1 by the queen.  The rook falls.  The proper tag is "Skewer" (more specfically it's an "absolute skewer") since a more valuable piece moves out of the way unscreening the attack, but clearly an X-ray attack is involved.

   

Álvaro Example No 1 -  White wins material The b7 square is directly attacked by the White's knight on a5, and X-ray attacked by White's bishop on g2. After 1.Nxb7 Black loses a piece, but could play 1...Bxg7, thus never exposing the b7 square to direct attack from White's bishop.  Of course, the bishop could just remain on d5 as well. If Black plays 1...Bxb7 the bishop falls to the uncovered X-ray attack (ie. the direct attack from the bishop), and Black still loses a piece. The X-ray attack plays a central role, and the problem most likely would be tagged "X-ray Attack" for this variation. But White could also win a piece with 1.Bxd5+ Kxd5 2.Nxb7, in which case the better tag would be "Removing the Defender".

Álvaro Example No 2 -  White doesn't win material This time the blocking piece is a knight, and Álvaro would not label this problem as involving an X-ray attack. In fact, he goes so far as to claim "but an *X-Ray Attack* through a Knight is something that does not exist". I disagree, the White bishop is still X-ray attacking the b7 square.  The problem with this example is that the position doesn't allow this attack to be unscreened or utilized.  No exploitation - no tag.  But just because a pin can't be exploited doesn't mean it's not a pin.  The same is true of the X-ray attack here. The next examples show small modifications to the position which do allow tactical combinations to activate the X-ray attack (i.e. promote it to a real attack or otherwise exploit it for tactical gain).

Modified Álvaro Example No 2a -  White wins material With the addition of a pawn at c3, White can now tactically exploit the X-ray attack and win material. After 1.Nxb7 Bxb7 2.c4 Black must either lose the knight or bishop.  The X-ray attack is critical to the gaining of material.  Whether the correct tag is "Pin" or "X-ray Attack" can be debated.  I would vote for "X-ray Attack" since pins are most strictly defined as involving unequally valued pieces, and because this isn't exactly a typical pin exploit. But regardless of what tag is used, the role of the bishop's X-ray attack is clearly revealed. And for essentially the same position as last example.

Modified Álvaro Example No 2b -  White wins material Here, after 1.Nxb7 Bxb7 axb4 Black loses a pawn since the knight cannot take the b4-pawn without exposing the bishop on b7 to the X-ray attack of White's bishop on g7.  Again, there could be a debate as to whether the tag should be "Pin" or "X-ray Attack" because the X-ray attack involves a blocking piece which is the same value as the target piece.  In the previous post I expressed a preference for "Pin" for this example, contradicting what I said above.  But heck, I'm only human after all, and we are a being full of contradictions - living in an imperfect world, alas!

Finally, since the dreaded involvement of the pin concept can no longer be avoided - I should update my taxonomy:

Code:

+---------------------------------+
|         X-Ray Attacks           |
|                                 |
|                                 |
|                                 |
|                                 |
|                                 |
+--------------+   +--------------+
|  Skewers     |   |    Pins      |
|              |   |              |
|              |   |              |
|              |   |              |
|              |   |              |
+--------------+---+--------------+

[OK - I've come back to rework, this edit is done (for now)]

--interlist

[richard]

While I agree with interlist that it is certainly possible to define x-ray and skewer in the subsetted manner he discusses, I think it is also equally easy not to do so by looking at the difference in focus and role on the piece "in front" and "behind". It is hard to avoid having some tags that are supsets of other tags, but it is usually more useful if they can be defined in such that a superset relationship does not exist. Using wikipedia as a reference in itself is tricky, because you are looking at the opinion of the last person to have edited the page. Here are some previous wikipedia definitions on x-ray that have been replaced by the current one:
- "In chess, an X-ray or X-ray attack is a tactic whereby a ranged piece supports a friendly piece "through" a piece of the opposite color. It is different from the skewer, the pin and line clearance, with each of which it is often confounded."
- "In chess, an x-ray or x-ray attack is a tactic that involves the action of a piece being felt "through" a piece of the opposite color. It is different from the skewer, the pin and line clearance, with each of which it is often confounded."
- "In chess, an x-ray is an attack that involves the action of a piece being felt "through" a piece of the opposite color. It is different from the skewer, although many writers confound the two."

Some of those survived some time before being replaced with the current version which mentions the skewer synonym usage (which I agree, you can definitely see in the literature, I have books in my own bookshelf that use the term as a skewer synonym). Anyway, I find a precise definition of x-ray quite confusing myself, and it is one of the tags I feel I don't have a great grasp on, so I'm happy to look on from the sidelines for now and enjoy the discussion. One thing I will say is that I agree that differentiating defence and attack x-rays would seem useful.

Regards,
Richard.

[interlist]

Beautiful instructional examples.

I finally looked at the latest examples Álvaro sent (CT 31790 / 54798 / 98318).  They are all truly excellent examples that were a pleasure to play over, and nicely illustrated both the offensive and defensive utilization of the X-ray attack.  

[interlist]

While I agree with interlist that it is certainly possible to define x-ray and skewer in the subsetted manner he discusses, I think it is also equally easy not to do so by looking at the difference in focus and role on the piece "in front" and "behind". It is hard to avoid having some tags that are supsets of other tags, but it is usually more useful if they can be defined in such that a superset relationship does not exist. Using wikipedia as a reference in itself is tricky, because you are looking at the opinion of the last person to have edited the page.

Here are some previous wikipedia definitions on x-ray that have been replaced by the current one:

- "In chess, an X-ray or X-ray attack is a tactic whereby a ranged piece supports a friendly piece "through" a piece of the opposite color. It is different from the skewer, the pin and line clearance, with each of which it is often confounded."

- "In chess, an x-ray or x-ray attack is a tactic that involves the action of a piece being felt "through" a piece of the opposite color. It is different from the skewer, the pin and line clearance, with each of which it is often confounded."

- "In chess, an x-ray is an attack that involves the action of a piece being felt "through" a piece of the opposite color. It is different from the skewer, although many writers confound the two."

Some of those survived some time before being replaced with the current version which mentions the skewer synonym usage (which I agree, you can definitely see in the literature, I have books in my own bookshelf that use the term as a skewer synonym). Anyway, I find a precise definition of x-ray quite confusing myself, and it is one of the tags I feel I don't have a great grasp on, so I'm happy to look on from the sidelines for now and enjoy the discussion. One thing I will say is that I agree that differentiating defense and attack x-rays would seem useful.

Regards,
Richard.

After reading Richard's post I dug into the background of the X-ray Attack article.  The three examples Richard quotes are indeed older versions - slightly modified from each other - but all making the "confounding" assertion that a skewer is not an X-ray attack.  Here is an excerpt of the discussion from WikiProject_Chess (it dates back to 2009, when were the examples Richard cited from?):

WikiProject_Chess:  X-ray (chess)

I am concerned that the article X-ray (chess) (and also Glossary of chess#X-ray attack) may not be accurate. The article claims "It is different from the skewer, the pin and line clearance, with each of which it is often confounded.", but offers no sources. (I tagged it as unreferenced in April 2007.) Brace (1977 & 1989, An Illustrated Dictionary of Chess) says that X-ray is another word for skewer. Horton (1959, Dictionary of Modern Chess) says that it is an expression used in place of skewer by Fred Reinfeld and some other chess writers. It may not used much in the UK as neither Golombek or Sunnucks include it in their chess encyclopedias. The evidence suggests to me that "X-ray" and "skewer" are synonymous, and further that X-ray is the lesser-used term. If so, X-ray (chess) should be changed to be a redirect to skewer (chess) and the definition in Glossary of chess#X-ray attack should be changed to "See skewer". Does anyone have any definitive sources that could help here? Quale  (talk) 07:35, 22 February 2009 (UTC)

    Hopefully Edward Winter clears this one up here [9] and here [10]. This supports the stance taken by Brace and others. However, there are many references on the net to this concept of "gaining increased control of a square through an enemy piece" and I must say, as a chess player, I find the term fits that particular attacking motif quite well, even though it is not a common term in the UK (as you pointed out). Perhaps the way forward is to simply describe its origins with the Winter refs and then go on to acknowledge its more recent 'hijacked' form, as a prelude to what is already in the article? Brittle heaven (talk) 11:16, 22 February 2009 (UTC)

        The term is not in the Oxford Companion to Chess or Burgess' Mammoth Book of Chess either. To me, an X-ray attack is a little more general than a skewer. A skewer wins the piece on the other side. Bubba73 (talk), 15:18, 22 February 2009 (UTC)

[...]

 Krakatoa (talk) 20:46, 19 March 2009 (UTC)

    I have now found sources supporting the usage of X-ray (in addition to as a synonym for skewer) as (1) attacking an enemy piece through a friendly piece and (2) defending a friendly piece through an enemy piece. I have supported the prior usage by referring to IM Orlov, IM Palliser, GM Rohde, GMs Kolev/Nedev, and Neishtadt's use of "X-ray" in that manner. I have supported the latter usage by referring to NM Pandolfini and IM Silman's use of "X-ray" in that manner. Bubba73 helpfully alerted me to Silman's discussion of the term. Krakatoa (talk) 19:40, 6 May 2009 (UTC)

As is apparent, the previous definitions quoted by Richard were unreferenced and had no sources. The current Wiki definition, has been sourced by Krakatoa, and appears to be stable for the last three years.

The WikiProject_Chess people also refer to Edward Winter, which is what I have done as well. From Winter we arrive at Harkness' definition, which is the definition I am arguing to support.  The term appears to be somewhat uncommon in the UK, which is a little surprising to me.  The discussion concludes, as I do, that X-ray is more general than skewer.

(Aside- the abstraction of X-ray Attack by some of the masters cited at the end of the quote includes action through pieces of both colors, friend and foe, which is also my personal definition in its broadest sense).

I think the Wikipedia chess pages are quite good (especially the rook+pawn endgame pages), but I've only been referencing them for about half a year.  Apparently there is a collaboration between Wikipedia and WikiProject_Chess, as cited on this page:

Wikipedia: Talk:X-ray (chess)

This article is within the scope of WikiProject Chess, a collaborative effort to improve the coverage of Chess on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.

Apparently, WikiProject_Chess is an effort to provide more uniform and better vetted articles on Chess for Wikipedia. Given the definitions Richard quoted above, I can understand the desire to clean up the treatment of technical topics such as this.

From the WikiProject_Chess homepage:

This is a WikiProject, an area for focused collaboration among Wikipedians.

Welcome to WikiProject Chess!

Some Wikipedians have formed a project to better improve information on chess-related articles. This page and its subpages contain their suggestions; it is hoped that this project will help to focus the efforts of other Wikipedians. If you would like to help, please inquire on the talk page and/or see the list below.

[...]

One of the goals of WikiProject Chess is the uniform presentation of chess related articles. For this aim, a couple of templates have been created. See Category:Chess templates for a full list; the most useful ones are explained below.

[...]

    * Top importance articles that require much work. Chess strategy, Chess tactics, Queen (chess), Rook (chess), King (chess), A.D Philidor, Boris Spassky

Of course I may be a little biased, but I hope I've provided some more evidence to buttress confidence in the Wikipedia definitions as they currently are written.
 

[munich]

(One thing I would like to say to tagging discussions: I and others do tag-sorted sets. We would like to see a lot of similar puzzles with the same idea/patterns. What we dont want is splitting academic hairs to allow as many puzzles under a tag as possible. The general rule should be: if in doubt (because the puzzle is highly untypical) then please better do not give the puzzle a tag, because tags are used to find puzzles by the tagging filter. Especially if you dont do a tag-sorted set, please be defensive and carefully when you tag a puzzle. Besides, if it was me, I would only allow silver/gold members the possibility to tag a puzzle (= "tagging" as a feature you need to pay for), since these people are more likely those who use tags as a training).

[alvarofrota]

Rio de Janeiro, 5 de setembro de 2012.

Prezado interlist:

I saw in the home page that you do not have a premium membership and, thus, you probably do not deal with "custom sets" and perhaps you do not fully understand the reason behind munich words. Thus, I will give you a glimpse of this kind of training. Imagine that you are training with a personal set that contains, say, 200 or 300 problems like the following ones:

53627
56606
29936
55386
53715
74379
64326
71869
25099
78642

I think you will agree with me that this kind of focused training will produces a great pattern recognition in your mind. (Yes, this is true: just yesterday night I delivery such a X-ray Quen & Rook mate in one tournament game at my club.)

But, to this kind of training becomes feasible, people must vote correctly to a "X-ray attack" Tag, strictly respecting the Chess Tempo definition.

It is correct to discuss the more generic definition and I agree with you in some of your points. But to tag accordingly the Chess Tempo definition is a necessity for practical reasons!

Aquele abraço!

Álvaro Frota

[richard]

Hi interlist,

I think I agree with Álvaro here. As I mentioned in my previous post, I think your definition is fully justifiable, I just don't think it is the most useful definition from a practical point of view on CT, as there are obvious benefits to keeping tags as mutually exclusive as possible. While it is clear that both historically, and in modern usage, skewer and x-ray are often used interchangeably, there are also more modern references that use the mutually exclusive approach. For example , The Pocket Guide to Chess by Jonathan Berry starts to define X-ray attack as mixture of pin and decoy, finishing with the statement "In one older book, an X-ray attack is the same as a skewer". He provides an interesting example on page 35 where a single moves applies a skewer in one direction, and an x-ray in the other, something he points out, which shows he sees them as two different things (you can see this page in google books). Pandonfini's Ultimate Guide to Chess seems to use a similar definition to Álvaro when discussing the term.

An online definition which matches nicely with your own is here:
http://www.chess-game-strategies.com/x-ray-attack.html

Basically they see skewer as a subset of x-ray as you do, and see x-ray as covering skewer, defensive x-rays and attacking x-rays. I think this is the neatest definition, but as mentioned above, forcing them into the three different types rather than creating an umbrella tag is probably the best fit for the way tags get used on CT. Furthermore, If I was to create an umbrella tag, I probably wouldn't call it x-ray  in order to side-step the types of discussions here. While one definition may have more historical weight than another, there is enough disagreement, both here and on wikipedia to try to find a less controversial name for the generic theme underlying all these types of skewer/x-ray positions.

Regards,
Richard.

[interlist]

Ironically, I've never suggested altering the actual tagging of any CT problem in this discussion - so far I've agreed with everyone of Álvaro's tags.  What I have been arguing about is the conceptual framework.  

As concerns tags I've argued that the specific should have priority, which means a skewer is only tagged as a skewer, and a pin as a pin.  I tried to preempt the concern about the tagging status quo from my very first post:

This might be better titled "X-rays and Skewers (Slight Return)", as there has been prior discussion on this topic: [...]
It is with some trepidation that I make this posting, arising from some comments between Alvaro and myself on problem 2682.  [...]

I want to empathically state, for the record, that I have come to understand and appreciate the emphasis many of the players on CT put on correct tagging.  And I don't believe there is any disagreement between Alvaro and myself that the correct tag for this problem is unequivocally "skewer".  

The controversy, such as it may be, is about nomenclature.  That is to say, whether or not my assertion that
all skewers are also X-ray attacks
is correct.  [...]

What precipitated this thread were emphatic and categorical comments which were most likely intended to instruct in the correct tagging of problems, but I perceived to be incorrectly telling me how to conceptualize the problem. Here is the original exchange:

alvarofrota [19th Apr 11 20:55] -
-----------------------
NOT AN X-RAY:
-----------------------
According to Chess Tempo Definition, an X-ray attack only occurs when one of your pieces attacks a square or a piece of the adversary *through* a piece of the adversary. Thus, an X-ray attack only occurs when your piece and this piece of the adversary have *the same line of action* in a file, in a rank or in a diagonal.
As nothing of that occured in this problem, I voted against this Tag. Despite that, it is still here.
Please, take a look at the Tactical Motifs page and aid Chess Tempo to maintain consistent our Tag System, voting down this Tag!
   
quoisme [2nd Nov 11 15:43] -
Thanks alvarofrota. I went to the motif page and found that the other explination is that the piece behind the attacked piece in a Skewer is a lesser value piece.

interlist [29th Aug 12 17:42] -
(Discretion is the better part of valor...)
I just read Winter's notes on this and am of the opinion that it is an x-ray (bQb7-wKg2-wRh1), but since it is also a skewer, and since skewer is more specific, it should be referred to it as such.
All skewers are X-rays, but should be tagged as skewer and not X-ray on CT.
(A statement almost rising to the level of legal logic - but please refer to @quoisme's comments for a better explanation).
See http://www.chesshistory.com/winter/winter20.html#4245._X-ray_attack_C.N._4231 and Note 4231 above it.
It is hard to argue with Edward Winter, his research is so thorough and accurate.


alvarofrota [29th Aug 12 20:42] -
Olá interlist!
Chess Tempo has a specific definition of *X-Ray Attack* that differs from other definitions that can be found in the literature:
"An *X-Ray Attack* occurs when one piece attacks a square or piece *through* another piece."
This means that an *X-Ray Attack* "is not the same as a *Skewer* as the relative value of the piece being attacked through is irrelevant."
More than this, in an *X-Ray Attack* the line of action of the pieces must be the same. For instance, a Bishop can *Skewer* a Rook against an undefended minor piece, but there is not an *X-Ray Attack* of a Bishop through a Rook. Also, a Bishop, a Rook or even a Queen can *Skewer* a Knight against an undefended Pawn but an *X-Ray Attack* through a Knight is something that does not exist.
And, at least in my view, an *X-Ray Attack* is a *Tactical Motif*, a condition of the board that can be exploited whereas a *Skewer* is a *Tactical Theme, an idea of piece action that depends, to be concretized, of the existence of a previous *Tactical Motif*.

interlist [2nd Sep 12 21:40] -
Olá Alvaro - right back at ya!
OK, this might get a little involved, and diagrams will surely help. I'm still trying to understand why you don't consider all skewers as a more specific sub-category of X-ray attacks.So, I'll see you in the forums for a more elaborate discussion if you wish to followup.
I think that we agree on the preferred tag being "skewer".
(oXo) (o=x=>o) || (o<=x=o) (oXo)

Again, it is clear that I am not asking to change the tagging of problems on CT.  I have been advocating for a conceptual framework with simple definitions of each term that are widely accepted and have accurate historical antecedents.  I thought I also presented the case for how this conceptual framework fit nicely into the current CT tagging practice.

I think I should formulate my next post as a series of questions in order to crystallize and focus the points of the argument where I think there is genuine disagreement (or confusion on my part about some of the counter-arguments).  I'll reread the thread before posting - so stay tuned.  

--interlist

PS- Thanks Richard for the link to http://www.chess-game-strategies.com/x-ray-attack.html.  
       It has nice graphics and I was going to reference it myself.

       Obrigado Álvaro for the links, I'll enjoy your problem selection.

       Und Herr Munich,  bitte mach dir keine Sorgen, Ihre Tagging-System ist sicher (bis jetzt)!  

[interlist]

In keeping with the high level of scholarly excellence of this thread - I have done some additional background research on the qualifications of two of the authorities previously cited as sources in prior posts.

First - the Wikipedia author krakatoa is in actuality Frederick Rhine, a USCF National master (1983) and USCF Senior Master in correspondence chess (1997) as well as a professional lawyer.  A selection of his games is available on chessgames.com, including two that have been included in past issues of Informant.  He is apparently a prolific online contributor on the subject of chess at both Wikipedia and at chessgames.com, where most readers would recognize him as fsr (aka "that mustached guy").  I myself have read enough of his comments to have acquired a deep respect of his knowledge (save for an anagram or two he has recently posted).

The site Richard referenced earlier, www.chess-game-strategies is written by Graham Wadden. It has a good description on tactics, and especially relevant is its treatment on X-ray attacks.  All the articles contain superlative graphics, and I also recommend it because his concepts follow mine. He is more of an expert web designer with a keen interest in chess, so it's not immediately obvious where he acquired his viewpoint.  But apparently, given his wholehearted recommendation of both of Seirawan's books "Winning Chess Tactics" and "Winning Chess Combinations" in the two online articles -- Chess Puzzles- Using Puzzles To Develop Your Chess-Brain and Chess Combinations- What I Learnt From Yasser Seirawan -- we must conclude that Wadden is largely following Seirawan's ideas.  (I will be reviewing library copies of these books in the near future to confirm this.)