Building control units are used to regulate the temperature inside buildings and are often called thermostats. These units are linked to heating and air conditioning systems and adjust the temperature at point of measurement; the location of the sensor must be selected to ensure even heating, particularly if only one thermostat is used in a domestic setting. The simplest type is a plug-in thermostat inserted into a wall socket outlet. These units, some of which have a small digital display, are fitted with a socket allowing the connection of an air conditioner or, more commonly, a portable electric heater. For centrally heated buildings, a room thermostat temperature regulator is used. Depending upon the design of the central heating system, this thermostat may directly control the heating and cooling unit. With ducted air conditioning, the thermostat may be connected to a temperature sensor placed in a duct to allow control of the temperature of a portion of the building using dampers. More sophisticated room thermostats timers incorporate a LCD screen and an internal controller that permits full control over when the central heating operates and at what temperature. Manufacturers of building control units include Elbro, Thermokon and Grässlin.