The SPC or "Strat Presence Control" is often called the fat control. The original intent of the SPC was to allow guitar players to boost the midrange of their single coil pickup to give it a sound similar to a humbucker. This is accomplished by boosting the gain in a broad mid-band fashion and reducing the highs. The result is a fatter, louder tone from any single coil pickup, but can also be used with a humbucker to increase your overall volume and midrange for a really fat tone that's great for slide guitar.