One of the circuits that control a light in multiple places a

In a house or on a stairway, it is sometimes necessary to turn the same light on and off in two or more places.
(1) One of the multi-ground control circuits for electric lamps
    is shown in Figure 2-26. It can be seen from Figure 2-26 (b) that the wiring of switches S2 and S3 is exactly the same. If
one light is controlled in three places, just cancel the switch in the middle; if one light is controlled in four or more places , then Just increase the number of
switches in the middle.