This chapter so far has concentrated on absolute pins—in other words, cases where a piece is pinned to its king. Relative pins arise when you pin one enemy piece to another that isn't the king but merely is more valuable than the target in front of it. These sorts of pins aren't as reliably devastating as the absolute variety because your opponent legally can move the pinned piece; it just is costly for him to do so. And they can be harder to see because there is no simple method for finding them that is comparable to scanning the king’s lines. But relative pins arise all the time and can be enormously consequential, so it's well worth your time to master the patterns involved. Our searching technique here will involve looking for enemy pieces on the same lines, and asking at all times what enemy pieces you attack (or could threaten) and—the key point—what lies behind them.
In the frame to the left, start with one of those simple questions: what do you, playing the White pieces, currently attack? Answer: you attack the pawns on b6 and e5 with your knight, and the knight on c6 with your bishop. We aren’t concerned at the moment with attacks you can make with your knight, because knights can’t pin anything; they jump rather than slide, so they can’t be aimed through one enemy piece at another. But bishops are superb pinning pieces, so when they attack something, like Black’s knight here, ask what lies behind the target. The answer is a rook. And although the rook is protected, it is worth more than White’s bishop, so White has a relative pin: Black’s knight is not quite paralyzed, but it would hurt him to move it.
What do you do with a pinned piece? The usual: throw a pawn at it if you can. White thus plays b4-b5, and now Black must choose between forfeiting the knight and losing the exchange. He probably will move the knight and suffer BxR, then recapture. In that case the "pin" admittedly doesn't function in quite the way to which we have become accustomed. The pinned piece moved, which is precisely what it couldn’t do in the positions where it was pinned to the king. The result here is you win the screened piece rather than the pinned piece. The logic of the outcome is similar to the logic of the skewer, which we will be studying in a little while. If the rook on a8 were unprotected, Black would have to let his knight go and the result would be a more classic relative pin.