Glass shade shapes have names, and learning a few of them turns finding the right shade from guesswork into a request. The common forms recur across a century of fixtures, and knowing what to call the shape you want makes the whole search easier, for you and for whoever's helping you find it.
The schoolhouse shade is the round or bell-shaped opal globe most people picture: a smooth, closed shape that surrounds the bulb and diffuses it evenly, at home in kitchens, halls, and pendants of many periods. The globe is a full or near-full sphere, enclosing the bulb entirely, giving a soft glowing ball of light. The bell is exactly that, a flared, open-bottomed shape like a bell, wider at the mouth than the fitter, which directs more light downward while spreading it at the edges. The cone and the dome cover the tapered and the rounded-top forms, the cone narrowing from a wide mouth, the dome curving over like a shallow bowl inverted. The saucer and the disc are the shallow, flat-leaning shapes used where a low-profile diffuser is wanted, often on flush and semi-flush fixtures. Knowing these names narrows a search fast, because the shape determines both the look and how the shade throws light.
Salvager's Note: Specify two things and the search gets short: the shape (schoolhouse, globe, bell, cone, dome, saucer) and the fitter size. Together they point straight to what suits your fixture, where "a glass shade" alone points nowhere.