If a filament goes open in the chain of lights, something that can't take much voltage goes short, and the rest of the chain stays alight. All the bulbs get a bit more voltage , and their lives get shorter and brighter. After a few have gone, the rate of failure becomes exponential. This is where the fuse bulb comes in, it doesn't have the thingy that goes short. So when it blows they all go out. The other option is the last few bulbs go off like chinese firecrackers and the wiring bursts into flame. NO, DON'T TRY IT! :twisted: