What is the fork?

Then your opponent is all forked up.

Kenneth Harkness wrote (in "Chess Life & Review" in 1947):
“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.”

One could also call it a skewer -although that might have connotations that aren't applicable.  I wouldn't consider it a pin except in the loosest sense.  As Harkness said, it's a double-attack:  one is direct, the other indirect. 

RedGirlz,

"a skewer, the attacked pieces are in a continuous line"  is exactly what's happening in #13.

The reason I added the delimiter that it "could also call it a skewer -although that might have connotations that aren't applicable" is precisely the reason you presented-- in a skewer, the more valuable piece is enticed to move and the real attack is on the less valuable piece behind it.

I maintain that "I wouldn't consider it a pin except in the loosest sense" because, while what you said is undoubtedly true and accurate, I don't see a pin as an attack in itself - and I could be wrong, but I see a pin as a controlling tactic.  Generally a piece that is pinned isn't seriously threatened to be taken - in fact, in most situations, if taken, it will result in a simple trade. One purpose behind a pin (besides limiting the other player's options) is to allow a second direct attack on that piece.  So the pin itself isn't a threatening attack but rather a possible initiation of a real attack. 

I also think my example was poor - not expecting to get into deep waters (although it's still an x-ray attack which is a double attack or an attack on 2 pieces which was my real point). Better might have been to reverse the Black N and Q, and remove the White Queen completely from the attack: 

So, yes, you are absolutely correct. But I hope I explained my reasoning effectively.

Well, a pin does threaten to capture and though that's sometimes a result, it's seldom it's underlying purpose.  It's real purpose is simply to prevent a piece from moving (for various reasons).  

I understand you point.  

It's rather fascinating to me that what might be construed as a simple, one-dimensional concept can actually contains several different facets that can be explored and discussed.

Thanks. 

Something that is used with a knife and sits next to a plate

This would also be a fork.

it is considered a discovery.

There are many forks, but only one can be considered THE fork.

Unfortunately White still gets mated in four moves.

A fork is not a pork

A FORK doesn't mean only one piece can escape. depending on the board situation, pieces can both escape by tempo, UNLESS its a fork that involves a KING. A PIN is a Threat where in a more VALUABLE piece is behind the threatened one, and reversely a SKEWER is when the attacked piece is valuable than the ones behind it. 

The Fork is a kind of Double Attack and the term is not exclusively for the KNIGHT's double attack, it is also same for all other pieces: they are called fork when a piece attacks multiple enemy pieces at once.

Every time I see this thread pop up, I instinctively read it as "What the FORK???"

This is the ultimate fork.

Less commonly seen in games (fork by the King).

That is the Fork Of All Forks!