We turn now to a different pattern: the rook that blocks and then unmasks an attack along a diagonal by a bishop or queen. In a way the pattern is the reverse of a bishop discovery; this time the unmasker moves vertically or horizontally while the attacker runs on a diagonal. Since the mechanics involved are familiar, we won't spend quite as much time on the rook as we did on the bishop or as we will on the knight. Still, there are some differences between the patterns that are worth a few moments of special attention and visual absorbtion.
First, an unmasking rook usually has a harder time giving check than an unmasking bishop does. The bishop can act as a dive bomber, coming in on a diagonal to attack the king from above or below; the rook has to deliver check by moving straight down onto the king’s rank, or (less often) over onto the king’s file. Either of these moves can unmask an attack, as we shall see, but they are less common than the bishop’s strike against, say, h7. The significance of this is that when the rook discovers an attack, the distraction it creates more often is not a check but instead is some sort of mating threat, typically against the back rank. And discovered checks—i.e., moves where the piece behind the rook gives the check, rather than the rook itself—also make up a relatively large share of rook discoveries. Finally, discoveries by the rook also look different than discoveries by the bishop. The rook kernel isn’t as easy for the untrained eye to see as the bishop kernel; the eye is accustomed to looking up and down the files but not so accustomed to scanning the diagonals in the same way. The positions in this chapter are meant to build that visual habit.
Let’s begin with positions that are comparable to the first studies in the chapter on bishop discoveries. There, a bishop masked a queen or rook that was ready to attack an enemy piece along a file; here, a rook masks a bishop or queen that is prepared to attack an enemy piece along a diagonal. There, the bishop unmasked the attacking piece with a diagonal move that checked the enemy king; here, the rook unmasks the attacker with a “vertical” move—a move down a file—that checks the enemy king. In either case, the unmasked piece has time to make its capture after the check is fended off.
Where does White have the makings of a discovered attack in this first example to the left? The answer presents a new pattern: it’s on the diagonal f1-h3, where the rook masks an attack by the bishop against Black’s queen. The questions about the idea's execution are familiar: what violent move can the rook make that will require a time-consuming response from Black, giving the bishop time to take Black’s queen? The answer, of course, is a check: Rxg7+ requires Black to play KxR (or move his king); and now White plays BxQ.