A bishop’s power—its potential to execute double attacks and its usefulness in other ways—generally depends on whether it has open lines on which to move. A bishop placed on one of the long diagonals with nothing in its way is a mighty force on the board; a bishop blocked by its own pawns has relatively little use. It might seem to follow from this that you should try to maneuver your bishops onto open diagonals, and indeed that generally is good practice. But there also are other, subtler steps you can take to increase the power of your bishops and limit the power of your opponent’s.
The first thing to understand is that at any given moment your two bishops may differ greatly in their attacking potential. One travels on the dark squares, the other on the light squares. Usually one of them turns out to be more useful than the other, because either the dark or light squares in the middle of the board—but not both—will be open. “Open,” here, means unoccupied by pawns. Notice that your pawns frequently are set up on squares of the same color; that is how they protect each other. If your pawns are arranged on light squares, those squares are “strong” for you and the dark squares are weak—i.e., unprotected by pawns. But it also means that your dark-squared bishop has room to run and that your light-squared bishop is likely to be less useful, at least until the pawn structure changes. Thus we speak of a bishop as good” if it travels on squares unobstructed by your own pawns; a bishop is “bad” if it travels on the same colored squares your pawns do. In the skeletal diagram to the left, both bishops are fianchettoed (i.e., White has developed them to the squares in front of his knights’ original positions). The dark squares are very strong for White because he controls them so thoroughly with his pawns. But a side effect is that the bishop on b2, which travels on the dark squares, is bad; the bishop on g2 is good.
We focus on the pawns near the center because the best diagonals pass through the middle of the board. If you look back at the studies in this chapter, you will see that the attacking bishop usually takes advantage of open paths through the center; it rarely is the case that there is a center pawn on the same colored square as the bishop that delivers the fork. There may be an enemy pawn there that is captured by the bishop, but there generally is not an allied pawn that blocks the bishop’s way. Notice that even one pawn in the middle can be significant, since it single-handedly blocks long diagonals in two different directions.
These points have several implications. First and most obviously, you should think of your two bishops as quite different pieces, and you should be much more willing to trade away a bad bishop than a good one. Second, pawns and bishops have an intimate relationship. Think of pawns as pylons that obstruct the paths of the bishops; every time you move a pawn you open one diagonal and block another, and this may be the most important consequence of such a move. Whether lines are open or closed will matter for your other pieces as well, of course, but all of the other pieces have the option of moving back and forth between light and dark squares if necessary. Bishops cannot, so they are especially sensitive to pawn placement; moving your bishop often does less to make it powerful than moving a pawn out of its way and onto a different colored square. If that can't be done, you need to maneuver your bishop outside the pawn structure. In any event, try regarding pawn moves as indirect bishop moves.
This principle is important from a defensive standpoint as well. If you move a pawn and the move opens an important diagonal—especially a diagonal leading toward your king or another valuable piece—you immediately strengthen the enemy bishop that travels on that diagonal. Conversely, a pawn move or exchange can have a powerful effect on the enemy if it creates an obstacle—or, even better, gridlock—on the squares where his bishop wants to move. This is particularly significant if he only has one bishop left. Locking your pawns with his so that his bishop can’t get through the center will tend to make his bishop impotent for so long as the pawn structure remains in place.