Line-voltage thermostats regulate temperatures of HVAC systems that require direct electrical power. Also known as inline thermostats, they typically connect to wires supplying 120V or 240V of power to the heating or cooling device. They have two or four thick wires coming out of the back that attach with wire nuts to wires in the wall. These thermostats are commonly used with electric baseboard heaters, attic fans, and fan coil units. Nonprogrammable thermostats maintain the same temperature until manually readjusted. Programmable thermostats adjust the temperature according to a series of programmed settings that take effect at different days of the week. Plug-in thermostats plug into a wall outlet and include a piggyback outlet that allows users to directly plug in portable heating or cooling equipment.