Growing up in a Cleveland suburb in the '60s-'70s, I remember our Christmas trees and their lights. The trees were big floor-to-ceiling affairs (our house had eight-foot-high ceilings), and the lights were like Dave's--twisted-pair leads, branch clips, plastic sockets (Bakelite?
), etc. Ours also had fused plugs, with fuses in both sides of the line. I don't remember the fuses ever blowing, but we did have to replace dead lamps every once in a while; the dead one was easy to find in those older strings due to the manner in which they were wired.
Then came the '80s, and our C7 strings were replaced by the newer style series-wired strings that went completely dead when one lamp burned out. No fuses in the plug in those modern strings with tiny bulbs, either; they may have been wired to blow the house fuses if there was a short in the string, or if there was a fuse in the string itself, it may have been a thermal one located in the plug, ahead of the small socket into which another string could be plugged. (The only problem with an arrangement like that is, of course, when the thermal fuse blew, it ruined the entire string, even if the lamps were perfectly good.) When I moved in late 1999 I decided to forget about Christmas tree lights altogether; for the last 13 years all I've been putting up in my apartment at Christmas time has been a 3-foot-high fabric tree, without lights. I don't miss the lights, but it is fun looking back on the lighting arrangements on the trees back in the day.