When you see a possible knight fork, a natural first question is whether the square your knight needs is protected by any of your opponent’s pieces. If it is, your attention turns to the guard of the square and whether you can get rid of it—or whether you really need to get rid of it. Perhaps you don't; maybe the protection that the piece appears to offer is an illusion, as is the case if the guard is pinned. A piece is pinned if it can't move without exposing the king or another valuable piece to attack. Indeed, a piece that screens its own king from attack is subject to an “absolute” pin and so cannot legally move. We will study pins in detail in later chapters, but this much is enough to help you see that sometimes a square that looks well-defended really isn't.