What is a pin?

The usual definition of a pin is "a situation where a piece cannot move without exposing a more valuable piece to attack". I feel that this is incorrect. I am, however, unable to come up with a definition that actually works.

My first example is the following diagram.

In this diagram, I would say that the knight is pinned against the pawn, because if the knight moves then the pawn drops. However, this does not fit with the standard definition of 'pin', because the pawn is less valuable than the knight. And I wouldn't exactly call it a skewer, because that would suggest that the knight wants to move away.

Another situation is one where a piece in pinned against a square, in which case the usual definition falls apart once again:

If the knight moves, then checkmate follows, but there's no "more valuable piece" involved.

So maybe we can define 'pin' as "a situation where a piece simply does not want to move"? Well, even that is not possible:

1.Ng5 is checkmate. So you can't really say that the knight doesn't "want to" move.

So what exactly is the definition of 'pin'? Of course, I'm talking about relative pins: absolute pins are very clearly defined.

Thanks for reading! :D

You are quite right, the pin is more accurately defined as a piece under attack by a piece that moves in a straight line (bishop, rook, or queen) and which cannot, must not, or should not move because it is blocking the attacking piece from capturing material behind it or occupying a square of importance behind it. (Or something like that). Examples like your last one show that something can look like a pin but not actually be a pin. If the piece which "cannot move" actually can move, whether this is because it captures or threatens something more valuable, then by definition it is not pinned. We might then speak of an "apparent pin" or a "false pin." For beginners we usually start with the "more valuable piece" definition and then expand to show the more subtle types of pins.

Where did you find that definition of pin? That certainly isn't mine. My definition pertains to a piece that is unable to move without exposing a square of greater value (whether that square holds a piece or not) to one of the opposing pieces attacks.

That definition is the one I find in tactics books and such. Wikipedia, for example, states: "In chess, a pin is a situation brought on by an attacking piece in which a defending piece cannot move without exposing a more valuable defending piece on its other side to capture by the attacking piece."

Anyway, thanks for the verification guys.

The definition I use:

" a pin is a situation brought on by an attacking piece in which a defending piece cannot move without exposing a more valuable defending piece on its other side to capture by the attacking piece."

For more insights and exercises about pin, check out my latest blog post: http://chesstrainerapp.blogspot.fr/2014/01/the-pin.html

Why do you keep reviving 5 year-old threads to advertise your blog?

Well, but here's the thing, the definition of a pin is accurate, you are just interpreting it wrong.

If a Bishop is attacking a Knight, and right behind the Knight on the same diagonal is a Rook, then you have the basic case of the Knight being pinned to the Rook.

However, let's say that I have a Knight on g6, and if I move it, it exposes the h7-square where Qh7 is now checkmate.  In essence, the Knight is "indirectly" pinned to the King!  If I move the Knight away from g6 (let's say f8 is occupied by a Rook), and there is a White Bishop on b1, White Queen on c2, Black King on h8, and no other pieces on the b1-h7 diagonal, and nothing but the King covering h7, then yes, the Knight is pinned to the King, and so yes, there is such a thing as a "Relative Pin" that involves the King.  The Knight sure can legally move, but it sure wouldn't be recommended until you move that rook away from f8 so that the Knight can go there.

Anyone notice what is wrong with the last example...?
Where is the white king?!

The case I don't get is what if a bishop attacks two rooks like so?

Both rooks obviously have the same value -- so is this a pin or a skewer?  

Thanks, but according to all the definitions I have read about pins and skewers, both always involve one or other piece being of "lesser value" (and the attacking piece and defending men must all be in a straight line -- diagonal or orthogonal (Google it! )).

I've cited two rooks as they are obviously of the same value -- so neither the usual pin nor skewer definitions would seem to fit.

I've now come to the working assumption that the chess world hasn't actually got a name for this situation (unless anyone can convince me to the contrary) -- hence suggestions of "kebab" etc. in this discussion.

Currently the definition of "skewer" (having researched on Wikipedia etc.) is that the piece behind the directly attacked piece is of lesser value; my suggestion therefore is that this might be expanded to "a piece of lesser or the same value" (in which case Gruber86's comment would become true). What do people think?

I would propose changing the skewer definition rather than pin, as pins are more common and so the definition of pin is more widely established.

(According to Wikipedia an "X-ray Attack" is synonymous with "Skewer", therefore also referring to pieces of differing value and so not consistent with this example.)

A pin is a when a piece prevents a piece from moving and a skewer forces a piece to move both  through an indirect line of attack

A pin is a very sharp move!

A skewer does not force a piece to move.  Your bishop may hit my Rook, and behind the Rook is an unprotected Knight.

I am not forced to move the Rook.  There could be two reasons for that.

In the first case, there may be no way to protect the knight with the rook, and losing anot exchange is the lesser evil to losing a piece.

The second case is that skewers and pinstall are defined based on relative value.  In general, a rook is worth more than a knight, but in any given position, there are specifics to that position alone, and there are cases where a Knight is more dominate and worth more than a Rook.  This is especially true if there are no open or semi-open files that are not blocked by lesser pieces.  For example, if neither side has a d-pawn, White has a pawn on e4 and Black on e5 and c5,and White plops a bishop or knight on d5, the d-file is ineffective for a rook, especially a black rook.

dont forget the difference between relative  (you loose a  piece of higher worth f.e Bishop pins a knight to a queen) and absolute pin (f.e Bishop pins knight to king and cant move because it would result in a check to the king)