Inventory White's threats. He has possible captures to make with his a1 rook, his g2 bishop, and his c3 knight. One of the knight’s captures—Nxb5—is particularly interesting because on first inspection it looks safe: the pawn currently is attacked twice and defended just once. But again you want to take account not only of how many times an enemy piece is defended but also of how the board will look if you take it and whether any worrisome patterns will then be in place. Here Nxb5 is safe only in an immediate sense. Picture it and you should see that it puts White’s knight and queen on the same file. This could be trouble, and it is: Black can play his rook from a8 to b8, and now the knight is paralyzed: if it moves, White’s queen is lost. Since the knight is pinned, the next question is which side can bring more force to bear on it. From the start Black attacks it twice and White defends it once; and White has no way to add any more defenders. (There is Ra5, but you must ask whether such a move is safe. Here it loses the rook to QxR.) So the pawn on b5 is not as vulnerable as it looks.