That’s basically the title For those of us who grew up in the '80s and '90s, it was a source of humor, but in reality, it was a complete disaster
Christmas lights are connected in series, that is, they’re connected to each other So if one bulb burns out, the entire circuit is interrupted
As the other commenter pointed out, the bulbs are connected in series if one burns out, the circuit is broken However, at one time at least before they were replaced by LEDs, these little bulbs could continue to work even if one burned out, thanks to a shunt jumper wire, which keeps the circuit closed in case of a broken filament Learn more about how they work The page also explains series circuits and why the whole chain breaks I haven’t been able to find out exactly when these bulbs started using shunts as standard, but they were invented in 1929, so there was no excuse not to include them in Christmas lights in the 1980s
It is impossible to manufacture small incandescent bulbs that can operate on 110 V or 220 V mains voltage However, the mains voltage can be divided by passing current through several bulbs in succession In this way, a light bar with 12 bulbs, each consuming slightly less than 10 V, can be connected to 110 V If one bulb burns out, no more current can flow through it and none will light up LEDs consume even less volts, usually less than 3 V Technically, it is possible to connect them in series, and some do For this reason, up to 50 LEDs are sometimes found per bar However, LEDs rarely fail, so they are not noticeable The most common method is to reduce the voltage of the bar with a power supply and then connect each element in parallel or at least group them in series