The answer is rather simple: traditional Christmas lights (I'm ignoring newfangled LED varieties) were typically connected directly to the mains power supply and wired in series like this: Only if the filaments of all the bulbs are intact will a current flow around the circuit; if one bulb breaks then the circuit is broken and all the lights go out. The reason the bulbs are wired in this, inconvenient, manner is that it's convenient for the manufacturer. Although the supply voltage is 230v (or 110v) the bulbs are rated for a much lower voltage. At home I have a string of 20 lights like this with 12v bulbs. This works because of the rules of series circuits . In my home lights there are 20 bulbs each with some unknown resistance R. The total resistance of the circuit is 20R and the entire circuit is a sort of voltage divider . The current flowing through the entire circuit is I = 230/20R and the voltage across any individual lamp is V= R * 230/20R or 230/20. So my 2