CRW:
That was an early 3-way wiring system, where The common terminal of the two three-ways were fed to the light and the 120 volts was fed to the two traveler terminals, when exactly it was made illegal by the code I am not sure But I have an old wiring book dated 1936 and makes reference to this circuit as not being allowed anymore, primarily because it switches the neutral, but what I dont like about it is the fact that it used to be used for garage power for a light at the side door and at the garage and of course they used Keyless fixtures and in one switching sequence(of the four possible) the light are off but both the pin and the shell are hot, it was convenient but very lethal on a damp day.
-Mark-
