These are bars you see under the strings of an electric guitar that sense the vibrations of the strings and send them to an amplifier as electrical signals. Single-coil pickups employ one coil of wire wound around a magnet to translate the string's vibrations into electrical signals. Using two coils in a pickup makes it a humbucker: It's called a humbucker because the way it's made reduces hum and noise while creating different tones. Some guitars can have just single-coil pickups, just humbuckers, or both.