A) The official schoolbook wiring digram of a 3 way switch circuit. A single light source can be turned on and off from two different locations. The arrangement is most common in corridors and bedrooms. The phase (current) can choose between two alternative routings. Binary take, 4 logical states. [Extension, just for the shake of pure switchboard logic, one can link up an infinite series of 3 and 4 way switches (hard wired 'chiasmus'). Just for added background color: a shunting yard display panel or anything suitably double tracked and interconnected. - One dark and one lighted room would probably make more sense than a very long corridor with an endless long row of light switches - one per ancestral portrait.)]