In a discovered attack, or “discovery,” one of your pieces moves out of the way of another, unleashing attacks on two enemy pieces at the same time—one by the unmasking piece and one by the piece unmasked. The enemy only has time to protect one of the threatened pieces. You take the other one. The diagram on the left shows the idea in skeletal form. If White plays his knight to f6, it gives check while unmasking an attack by his queen against Black's queen. Black only has one move to respond to these two threats, and he has to spend it moving his king to safety. Then White plays QxQ. This at least is one of the patterns (a knight discovery) in its classic form; there are many variations on the theme that we will consider in due course.
Discovered attacks always involve two offensive pieces: an unmasked piece and an unmasking piece. Every piece has the power to unmask attacks by others by moving off of lines that it occupies. Not every piece has the power to be unmasked, though; a knight, for example, can't be unmasked because it can't be masked in the first place: it jumps rather than slides, so it doesn’t move along a line that can be temporarily blocked by a fellow piece. But the knight is a magnificent masker and unmasker of attacks by other pieces. Conversely, the queen is a great piece to unmask, but not a good masker of other pieces. It can’t hide an attack by a rook or bishop because a queen can make all the same moves that either of those other pieces can; if a queen masks a threat by a rook, it already makes the same threat the rook would. The essence of a classic discovered attack is that before it is executed, neither piece directly threatens anything. After it is executed, both of them do.
The plan of this section will be to take each of the major unmasking pieces—the bishop, the rook, the knight, and the pawn—and study one by one how they can unveil attacks by other pieces: what the unmasking piece looks like when it is poised to do this, how the germ of such an opportunity can be created, and how such ideas can be perfected and executed once they come into view. We will identify the visual patterns that signify the possibility of a discovered attack and practice identifying them until it becomes habitual.
Mastering discoveries means learning new ways to think about the pieces and the relationships between them. You may be accustomed to thinking of bishops as pieces that attack diagonally and to regarding rooks as pieces that attack back and forth and from side to side. That’s not wrong, but it’s incomplete. Bishops attack diagonally and unmask vertical and horizontal attacks by rooks and queens. Rooks attack vertically and horizontally and unmask diagonal attacks by bishops and queens. Make it one of your goals to think of your pieces not just as individuals but as partners—as parts of a team whose efforts need to be coordinated. Discovered attacks are an example of coordination, as each partner makes the other more powerful; a bishop and rook on the same file, with the former masking the latter, often has far more destructive power than either piece by itself.
Every discovered attack starts with a kernel consisting of two pieces: the piece to be unmasked and the piece that will unmask it. Once found, a kernel can serve to organize the rest of your thinking about what to do: you start looking for targets for each piece or ways to clear the lines between the pieces and their targets. We will be studying discovered attacks one kernel at a time: first the one where the bishop masks a rook or queen on the same file or rank; then the kernel where the rook masks a bishop or queen on the same diagonal; and so forth. We will emphasize spotting the kernel of a discovery because the practical importance of training your eyes in this way is so great. If you don’t see the basic pattern when it's there, all the skill in the world at perfecting it won't be of much use.