In the US and Canada, an electric blanket is a blanket with an integrated electrical heating device usually placed above the top bed sheet. In the UK and Commonwealth, electric blanket commonly refers to an electric underblanket (electric mattress pad), which is placed below the bottom bed sheet. Electric blankets usually have a control unit which adjusts the amount of heat the blanket produces. Blankets for larger sized beds often have separate controls for each side of the bed. The electric blanket may be used to pre-heat the bed before use or to keep the occupant warm while in bed. Some modern "low voltage" electric blankets have thin carbon fiber wires and work on 12 to 24 volts.