Why do my C9 bulbs keep burning out halfway through the season?

If part of the string stays lit while other bulbs are dark, you've likely got a parallel-wired string, which is common on the nicer C9 sets. On those, each socket gets full voltage on its own, so one dead bulb doesn't take out its neighbors. That also means a dead socket points to the socket, not the circuit. The filament looking fine doesn't clear the bulb, by the way. Incandescent filaments can break with a hairline fracture you can barely see, especially near the supports where you can't get a good look. The easy test is to take a bulb you know is good, screw it into the dead socket, and see if it lights. If it does, the original bulb was toast. If it stays dark too, the socket is your problem, and nine times out of ten that means the little metal contact tab at the bottom has gotten pressed down flat or corroded. Unplug the string first, then GENTLY pry that tab up a bit with a small screwdriver so the bulb's center tip actually touches it. Loose seating is the other culprit, C9 bulbs need to be screwed in snug, not just barely turned. On the daisy chain question, no, stacking strings doesn't overwork the bulbs. Every string plugged into that extension cord sees the same household voltage no matter how many are chained up. The real limit on chaining is the amp rating of the cord and plug connections heating up, and with two strings you're nowhere near it. Bulbs burning out fast is usually just cheap bulbs, vibration from people brushing the tree, or a surge from something big cycling on the same circuit, like a furnace or fridge.

One more thing, buy a spare pack or two of the same bulbs now. Incandescent C9s die in batches, and if all your bulbs are from the same box, the ones that survived week one tend to go around the same time later.

I run C9s on the house every year and the thing that fixed my constant bulb swapping was swapping to LED retrofit bulbs on the same strings. Kept the retro look, sockets and all, but I haven't pulled a dead bulb in three seasons since. They cost more up front but I got tired of buying replacement packs every December.

I had the same mystery last year with my C9s and it turned out half my bulbs were just loose. Big bulbs are heavy and if the tree gets bumped or the cat brushes past, they wiggle loose a bit and the contact drops. Now I give every bulb a firm quarter turn when I first hang them and maybe once mid-season. Also, don't toss the 'dead' ones right away. I've resurrected a few by just screwing them into a socket I know works. Sometimes they were fine all along and the socket was the issue.

Update: went back through the string and it was the loose bulb thing from the third reply. Two more were barely seated, gave them all a firm quarter turn and the three dead spots came back on. No burnt filaments at all. Still keeping an eye on them though.