Figure[White to move]

Using the Priority of Check.

As we have seen, one obstacle to driving off a queen with a threat is that a queen usually can capture anything that attacks it. In one common scenario your opponent’s queen is guarding one of his pieces, so you attack his queen with yours. Assuming that both queens have protection, how does he react? Not by moving his queen and leaving its protectorate loose, but rather by taking yours with QxQ. After you recapture his queen, the target the queen had been guarding—the piece you had been hoping to take after driving off the queen—simply moves, and your sequence is ruined.

The underlying problem when this happens involves move order. When you threaten the enemy queen and it replies by capturing your attacker, you've lost the initiative; now you are responding to his moves. But sometimes this trouble can be avoided by use of the priority of check: the requirement that a pending check must be addressed before doing anything else. We've seen the principle from time to time in earlier sections. Here we consider one application in detail: cases where when the capture you want to make in the end (after you drive off the enemy queen) also will give check. For then when the enemy queen takes the piece you used to attack it (e.g., your queen), you don’t recapture; instead you take the target you originally hoped to capture. Since this gives check, your opponent has to pause to save his king, and then after this you still can go back and take his queen. All this will be clearer after some illustrations.

Begin by taking the position on the left methodically. What does White attack? His bishop can take Black’s knight on h6, but it's guarded by Black’s queen. White can attack the queen with his bishop or rook, but however he tries to do this the attacking piece gets taken: Rd1 leads to QxR+, for example (and anyway Black could just move his queen to another square where it still protects his knight). The key point to see is that if White ever does play BxN, the result is a check of Black’s king. Of course Black can just move his king out of it, but this will cost him a move. In a sense this means that after playing BxN White will get a free move—two captures in a row. How to take advantage of this? By starting with the attack Qc7. Notice that this threatens Black’s loose bishop as well as his queen; Black thus cannot afford to just move his queen to a square like d6, as this loses a piece. So suppose he plays QxQ. Black might be expecting White to recapture RxQ, an exchange of queens that still gives Black time to save his knight. But now the priority of check takes its bite. Instead of RxQ, White plays the capture BxN+. Black has to move his king. Then White plays RxQ, recovering the queen and gaining a piece with the sequence.

Observe the elements of the pattern: a target guarded by its queen; a potential capture of the target that also will give check; a way for your queen to attack the defending queen with protection; and another threat your queen can make along the way (this time to Black’s loose bishop) that requires your opponent to play QxQ rather than move his queen to some other square from which it can protect the piece you are trying to take. Not all of these elements must be present in every case, but each of them does important work here.

With those basics out of the way, let’s also consider a couple of loose ends. After 1. Qc7, QxQ, White doesn’t have to play 2. BxN in order to win a piece; if he plays RxQ right away, he now has attacks pending against two loose Black pieces—the knight on h6 and the bishop on b7. Black only has time to move one of them, so White will be able to take the other. The danger of this approach is that Black can try to save himself by making trouble with either of his threatened men; since he isn’t in check, he has much more freedom in replying to his predicament. Here he can play Nxg4, picking up a pawn to help compensate for his forthcoming loss of a piece. The main line sequence described earlier doesn’t allow Black this luxury. It also wins White an extra pawn at the end: 1. Qc7, QxQ; 2. BxN+, Ke8; 3. RxQ, Be4 (saving the loose bishop); 4. Rxa7.