In the real, physical world, a “nail” is a small but sharp thing. In the chess world, a “nail” is not a common term, but it has been used to describe a pawn that is lodged deep in the opponent’s camp, near their king. It is blocked and immobile, but a major danger to the opponent. Here is an example:
Here the black pawn on h3 could be called a “nail”. I think Pyotr Romanovsky used this term to describe such a pawn in his book “Chess Middlegame Planning”, although that book is in a bin in my friend’s attic several thousand miles away, so it is hard for me to check.
Whatever you call it, the presence of such a pawn and its value/danger is something an experienced chess player should feel intuitively. About the above position, Kasparov said “In fact, the pawn on h3 can be seen as a material advantage for Black, because it is so important that you could value it as a whole piece. It not only helps the queen to create mating threats, but in most endgames, this pawn will also guarantee Black a winning edge because of the threats that Black can create against the h2 pawn, when the black h-pawn is very close to the promotion square.”
This is an important theme in chess. The pawn on h3 is a permanent positional advantage. Although an attack on the king is generally regarded as a temporary thing, this pawn on h3 provides a permanent lasting threat to the white king, sort of a Sword of Damocles. Black has no real attack on the king yet, but you know that as the game approaches move thirty, move forty, something is going to happen.
Furthermore, as Kasparov said, this pawn on h3 is a positional advantage for another reason, which it is easy to underestimate – although it is not a passed pawn, its promotion is a great danger as the game goes on. This kind of pawn should be regarded as almost “half-passed”. It is not hard to imagine sometime deep in the endgame that Black will sacrifice a piece on g3 to make the pawn passed (and promote immediately). Or the dark-squared bishop will swing around to g1 and removes the fixed h2 pawn. Thus this little pawn remains a standing threat for the entire game. Its very presence will constantly hinder whatever White tries to do throughout the game, even on far away parts of the board.
It is customary for us to think of attackers against the king as pieces – minor pieces, the queen, and the rooks – while seeing the pawns as something that are used to clear away the opposing pawns, or to be sacrificed to open lines. However, a “nail" on a square like h6, g6, or f6 can be just as much a danger to the opposing king as a bigger piece. Underestimating such a nail almost caused me to lose to a player rated over 650 points less, in a recent tournament:
Here I am up a full piece against a player rated under 1800 in the first round of my first tournament back in Europe this year. Sure he has a knight and the f6 pawn in the general vicinity of my king, but my bishop can come to g6 and everything is easily covered, while he cannot really bring up new attackers. It is true that the position is completely won. It is understandable (although not justifiable) that I let down my guard here, and was mostly thinking about getting him to resign in time to go for a walk while it was still light out. Without thinking at all I played 24…gxf6?!. There is no reason at all to open up the king’s position, although the position is still fully won. The simplest was 24…h6, just driving the knight away. After 25.exf6 I slightly regretted my last move and realized I had better think for a second, because it is possible to blunder here. At first I was going to play 25…Rg8, but got a little spooked by the fact that my king has no squares (although this is the simplest way to win and White has no special tactics here). As has been usual recently, my head was filled with total confusion. After a minute of daydreaming, I thought “this is idiotic, just play a move and finish the game, there is nothing to be worried about.” And I quickly glanced at 25…d4 26.Nxe6, saw that White had nothing, and played 25…d4??. Immediately after moving, my stomach sunk when I saw what I had overlooked (or rather simply neglected to calculate). Now you try to find it too.
The worst thing was that I realized that if he finds this one move, finding the rest of the moves to win would be well within the capabilities of someone rated even much lower than him. I spent an agonizing few minutes while he thought of his reply. I even considered offering a draw – but no, that would only alert him that there was something there. Finally, to my great relief he played 26.Nxe6??, which was met by 26…fxe6 27.Qh6 Qc7, and I won shortly.
It may seem that this was a fluke occurrence in which I let down my guard in an easily winning position, but in fact such things have been happening in every one of my tournaments for more than half a year. I think I would normally be careless in such a situation, but I would not blunder like that prior to last fall. The difference is that recently my chess games are barely real for me, and this total decline of concentration has made me almost unable to play chess since last August.
In any case, you can clearly see the enormous power of the pawn on f6. Even with a piece more, the move, and many defenders available, it was still possible for Black to lose. The presence of the pawn made a sacrifice which would normally be totally harmless completely deadly.
Now try solving the following positions. In each one somebody makes good use of that sharp, rusty, and possibly tetanus-infected nail that is stuck in their opponent's house.