A pin occurs in classic form when one of your pieces is aimed at the enemy king with some other enemy piece blocking its way. That other enemy piece cannot move, because if it does it will expose its king to attack. (In the skeletal diagram to the left, Black’s knight is pinned by White’s bishop.)
The paralysis of a pinned piece has several consequences. The most important is that it becomes a vulnerable target. Normally when you threaten one piece with another, the target piece has the option of running away. This option is lost when a piece is pinned; it is trapped on its square. The only way to protect it is by rallying other pieces to its defense. Thus the usual next step after a pin is created is to go after the pinned piece with other artillery—preferably something small but lethal such as a pawn, but anything less valuable than the pinned piece may make a fine attacker. (And sometimes the piece that imposes the pin will itself perform the capture.) Often a series of pieces from both sides will rush to the scene of the pin; in that case the outcome depends on how the number and value of the pieces attacking the pinned piece compare to those defending it. Time is of the essence in these situations, since as moves go by it often becomes possible for the enemy king to move and thus to release the pinned piece from its predicament.
The situation just sketched is the most common and important pinning pattern, but there are others as well. Sometimes the most important consequence of pinning a piece is that it can do no defensive work. And a piece can be pinned to its queen instead of its king; indeed, a piece can be pinned to anything more valuable than itself—or even to a piece less valuable than itself that is unprotected. When a piece is pinned to its king, this is known as an absolute pin: the piece cannot legally move. When a piece is pinned to some piece other than its king, this is known as a relative pin. A piece subject to a relative pin can be moved without compunction if the benefits of doing so outweigh the costs; a relatively pinned piece may move and inflict a check of its own, for example, buying time to move the now-exposed piece out of danger a move later.
This part of the book follows the same pedagogical approach as the others. In each of them we take a tactical motif, identify distinctive features of it, and then drill the importance of spotting those features on a crowded board and building tactical sequences from there. Thus in studying knight forks, one key was to realize that the potential for such a fork can exist when two enemy pieces lie on the same color square as one of your knights. In studying queen forks, the key was to look for a check the queen could inflict or a loose piece it could attack. If both could be found, the question was whether they might be combined; if one element could be found, the question was whether the other might be created. In studying discovered attacks, the key was to look for the kernel: one of your pieces blocking the path of another. Then we went to work looking for ways to create targets for each of the pieces.
Here our approach is similar. We begin with the absolute pin. The one thing all such pins have in common is that the enemy king lies at the end of them. This principle will guide our inquiries. In each position we'll start by tracing lines out from the enemy king, looking for any enemy pieces that are aligned with it or that might be drawn onto the same line and pinned there. This is a useful method both because it makes the process of hunting for pins more systematic and because it builds the good habit of always studying the enemy king’s position carefully.
We will introduce the relative pin and the skewer in due course.