The schoolhouse light shows up in more old buildings than any other fixture, and it's the plainest of the lot. A round or bell-shaped opal glass shade on a simple fitter, it hung in classrooms, yes, but also in kitchens, hallways, porches, shops, and factories for the better part of a century. It earned that reach by being nearly perfect for the job.
Why it worked everywhere
The design is almost nothing, which is its genius. A globe or bell of white opal glass diffuses a single bulb into soft, even, glare-free light. It mounts on a standard fitter, takes a standard bulb, and suits almost any room. There's no ornament to date it hard, no fragile detail to break, no style statement to fall out of fashion. It lights the room evenly and kindly, and it did so across styles and decades that agreed on little else.
Opal glass is the whole trick. It's white all the way through, not painted, so the light comes out warm and diffuse rather than harsh, and it doesn't chip its finish because there's no finish to chip. A schoolhouse shade that survived a hundred years of use often looks nearly new, because the material ages so well.
Getting the fit right
The variety hides in the shapes and the fitters. Bell, globe, tapered, ribbed; the fitter opening comes in a few standard sizes, and that number decides what a given shade will fit. The fixture never went out of style, because it was never quite in one. It just worked, everywhere, and it still does.
Salvager's Note: A schoolhouse shade fits by its fitter, the diameter of the opening where it attaches, not by its overall width. Measure that opening (2¼ inch is the most common) before you buy, because the fitter is the only number that decides fit.