It is an electrical circuit that is capable of opening and closing the two switches. To open one of the two switches on or off switches anywhere it. It noted the installation of stairs and into a house or a building, make it easy to use.
A circuit that can open and close the two switches. One switch to turn on or turn off the switch, two at one time. Therefore, installation of stairs and into the house or building. Ease of use
A circuit can be turned on and off at the switch two. To open or close the switch a switch two somewhere. So popular is installed at the stairs up and downhill of the house or building, convenient to use.