Thus far we have been considering positions based on a single root idea: a bishop masking an attack by a queen or rook on the same file. The masked piece travels vertically up the board after the bishop vacates its square. But of course bishops also can unmask attacks by pieces on the same rank—attacks that are horizontal, with the unmasked piece running sideways across the board. These are somewhat less common than vertical bishop discoveries; it is easier to arrange a vertical discovery because it's easy for a queen or rook to move from their starting positions on the back rank onto open files where bishops sit—and then to travel up the files once the bishops move out of the way. A horizontal discovery usually requires that a rook or queen get out toward the middle ranks of the board first so that it can then travel sideways productively. At any rate, the logic and mechanics here are no different than in the positions we already have studied. The main new challenge lies just in spotting a new kernel: the rook or queen alongside the bishop on the same rank, rather than behind it.
In the example to the left, where does White have the makings of a discovered attack? His bishop on c2 masks nothing on its file, but on the second rank it masks an attack by the White queen against Black’s rook. From here the thinking is familiar. The bishop must vacate the rank violently, and this it does with Bxh7+. After Black fends off the check with KxB, the way is clear for White to play QxR. White wins the exchange and a pawn with a simple discovered attack turned on its side.