On dual-control switch is a single dual-control switch. Single: one is a button on the switch panel. Dual control switches: two switches control one light on the line. Simply put is a lamp requires two switches to control, no matter which switch you use, it can make light is lit or extinguished.
For example: door with a lamp bedside switch common control room, upstairs and downstairs switch common control staircase lights, are used on dual-control switch, so the two switches can control lights. Both switches can be placed in the upper and lower ends of the stairs, or both ends of the corridor, so you can turn on the lights before you enter the corridor, through the corridor at the other end lights both lights, and avoid walking on the light eternal waste of electricity.
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