Knob and tube wiring
Unlike modern wiring, knob and tube supply and return wires were not always alongside each other. In the oldest installations, several branch circuits could share the same neutral wire, which can cause overloads. To prevent overloads in this layout, the neutral wires were protected with fuses. If a neutral fuse blew or was removed, it could create a dangerous condition where the power appeared to be off, but was in fact still present. Three-way switches could also be wired in such a way that the outer ring of a lamp socket, which is normally connected to neutral, could be connected to hot, which is an electric shock hazard.