Now suppose you study the enemy king’s position and see that you are close to being able to mate but are stopped by an enemy piece that guards the square you need to reach. The same procedures apply here as in the cases just examined. Turn your attention to that enemy piece and ask if it protects—or seems to protect—anything else that you might be able to take.
The most basic mating pattern involves two pieces—frequently the queen and some other—aimed at a square next to the enemy king; the queen lands there with protection and the king has no escape. In the diagram here Black has a classic formation for such a mate on the g-file, where he threatens Qxg2#. All that prevents this is White’s bishop on f3. So focus on the bishop: since it is committed to the protection of g2, it is incapable of performing defensive work elsewhere. Ask what else it appears to protect, just as you would if it were pinned, and you are led to the knight on e2. The knight effectively is loose. Black can take it with his e8 rook, gaining a piece.