Since we have been considering positions where a bishop unmasks check by a rook or queen, let's look at one especially important application in detail. The unmasking bishop need not always go hunting for an enemy piece to capture; it also may be able to move to a diagonal where it can contribute to checkmate. The key mating pattern we will examine is this: a rook attacks the enemy king along its file, with a bishop (a) providing protection for the rook, and often (b) helping to seal off the king’s flight squares. Here we will look at some examples of how the pattern looks in practice.
In the leftward position Black has the makings of a discovered check on the f-file. He can go looking for White pieces to take with his bishop, but the better plan is to move the bishop to a diagonal where it, too, can be brought to bear against White’s king. Hence Bd3++—a case of double check where two of Black’s pieces attack White's king at the same time. In response to a double check the only legal reply is to move the king. Here it must go to e1. Now comes the classic mate: Rf1#. See how the rook makes the first rank and the f-file impossible places for the king to be, while the bishop on d3 (a) protects the rook when it lands on f1 and (b) closes off the king’s other possible flight square, e2.