We have seen that if you want to spot and create double attacks for one of your pieces, it helps to have a clear understanding of what its natural targets are. In the case of a bishop fork (pictured to the left), as with queen forks, the most common targets are the king at one end of the double attack and a loose piece on the other. There are exceptions, of course, and we will consider them, but this is the most usual pattern and the most important to master.
The reasons why this pattern is most common are by now familiar from our other work on double attacks. Consider the impediments to a fork by the bishop—the reasons why attacking two pieces with your bishop might not work. One of the forked pieces might be able to capture the bishop if the bishop’s square isn’t protected; one of the forked pieces might be able to break out of the double attack by making a separate threat of its own; the more valuable or less protected of the forked pieces might be able to move, leaving at the other end a piece that has protection (or moving to protect it). But all these possibilities tend to be reduced when the king is one of your targets: your opponent is required to address the threat rather than saving the piece at the other end of the fork; nor does he have time to use the piece at the other end to make a counterthreat; and because of the king’s limited mobility, it usually cannot fight back itself against the forking bishop or launch a counterthreat of its own. And of course a loose piece at the other end of the fork is ideal because it can be won for free. It just can't be a bishop, for the same reason a knight is an unsuitable target in a knight fork: it can bite back against its attacker.
Our method of finding double attacks by the bishop, then, generally will resemble those we've developed elsewhere: searching for checks the bishop can deliver and pieces it can attack at the same time; and working to expose the king and loosen enemy pieces to create chances for those checks and attacks. The methods will be easy enough if you have read the chapter on queen forks. Whereas a queen sometimes may be able to check an enemy king in four or five different ways that need to be considered, the checks a bishop can make usually are limited to one and never can be more than two; and since each bishop runs on squares of just one color, only one of your bishops can possibly give check at any given moment. Examining a bishop’s checks therefore is quick and easy.
Indeed, you can think of a bishop as a little like half a queen; it can make the same diagonal moves a queen can make, but not the horizontal and vertical type. Every move we will see a bishop make is a move a queen also could make. But not necessarily a move a queen would make; for the bishop has the advantage of being worth less than the queen, and thus easier to sacrifice. Giving up the bishop to win a protected rook makes sense, whereas giving a queen for the purpose does not. We can use this point to add to our list of targets for forks by the bishop: not only kings and loose pieces, but also rooks whether they are loose or not. A rook always is a good target for a bishop, because (a) it is worth more and (b) its pattern of movement makes it unable to strike back at a bishop that attacks it.
Some bishop forks require not only that the forking square be available, but that the bishop be protected. Such protection generally isn't important when building knight forks because their targets (kings, queens, rooks, bishops, pawns) don't move in ways that allow them to strike back at a knight. Protecting the forking square also is unlikely to be important when creating a queen fork; if your queen can be taken, the fork probably won't be worthwhile regardless of whether you can recapture the piece that takes it. Bishop forks differ from those types because sometimes a bishop fork will include the enemy queen at one end of it. Since queens can move like bishops, a queen always can capture a bishop that attacks it—unless the bishop is protected. So this is the last point about the targets for double attacks by bishops: the target can be a queen if the bishop has protection.
To sum up, double attacks by the bishop generally involve some combination of these targets, listed in rough order of frequency: the king; a loose knight; any rook; and the queen if the bishop has protection. (Loose pawns are fine, too, of course.) Since some of the techniques for making a bishop fork work are similar to the techniques shown in the chapter on queen forks, the coverage of them here will be a bit more brief.