Okay, I think I get it. They're both electrically equivalent to this circuit: White main --- Bulb ----+ | Black main --- Sw

Okay, I think I get it. They're both electrically equivalent to this circuit: White main --- Bulb ----+ | Black main --- Switch --+ But "equivalent circuits" are not actually equivalent when you're an electrician. It makes a big practical difference whether the main goes to the bulb, with the switch as a spur (UK), or whether the main goes to the switch, with the bulb as a spur (US and others). P.S. Everybody please give pjc50 the karma I promised.