The incandescent century
Electric tree lighting grew up on screw-base incandescent bulbs sized by codes collectors still use: the small C6 of early strings, then the larger C7 and C9 bulbs that defined mid-century porches and trees. Vintage figural bulbs (fruit, Santas, lanterns, painted glass from the same factories that made ornaments) are a collecting field of their own, and dead strings are bought for the bulbs alone.
Bubble lights
The mid-century showpiece: a sealed vial of liquid over a bulb, bubbling merrily once it warms. They became a sensation in the late 1940s, faded, and never quite died; originals circulate on the vintage market and modern reproductions are made new. Working originals are charming and thirsty; treat cords and sockets of any vintage electric decoration as the safety project they are.
Minis, LEDs, and the smart era
Mini lights took over trees from the 1970s onward on price and cool running. LEDs then rewrote the economics: a fraction of the power, cool to the touch near heirloom glass, and effectively immortal. Smart strings (app-mapped, per-bulb color) turn the tree into a display surface; they are spectacular, and they are electronics with electronics' lifespan, not heirlooms.
Honest safety, briefly
Vintage electric decorations are unlisted, old-insulated devices. Enjoy them like collectors do: test carefully, replace suspect cords, never leave vintage strings running unattended, and let the LEDs do the overnight work.