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quote; Come on now plumber. You know he meant 3 way circuits
In that case it would have been "3 way circuit", since there is only ONE common/ordinary three way circuit. It just has many, not a myriad, variations of the one circuit, depending on where the switches are located relative to each other, where the power enters the circuit, and where it leaves to go the fixture. But when you break them down to the schematic, they eventually are all the same. This discounts speciallized systems with proprietary circuitry and switches. These have to be wired according to the manufacturers specifications and use their exclusive switches.