It is very often necessary to turn off the light in the room from the hallway or corridor. I must say that this problem can be solved fairly well-known three-wire electric circuit; however, it requires ispolzovaniya rare in everyday life four-pole switches. Meanwhile, there is an extremely simple method of manufacturing a switch-backup. However, he was fit only for the case when the primary and secondary switch can be installed on the same wall, but only from different points of view. Look at the picture. Here, the left rocker switch switches the electric circuit. And the right, the same device is connected as shown in the figure, nylon thread or fishing line with a main switch — mechanically tracks the movement of the primary key.