The SPC or "Strat Presence Control" is often called the fat control. The original intent of the SPC was to turn the tone of a single-coil pickup into a dual-coil. 

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 you can also use it on a humbucking to give you a high output, really fat tone that's great for slide guitar.

• Boosts gain in midrange
• Reduces highs
• Works with single-coil and humbucking pickups
• Output jack and battery clip included