The Coney Island hot dog is decided by the sauce, not the frankfurter. A beef hot dog goes into a soft steamed bun, and over it goes a fine-ground, beanless, loose meat sauce, then a scatter of raw chopped onion and a stripe of yellow mustard. The sauce is the whole identity. It is cooked down to a texture closer to a wet rubble than a chili, smooth enough to cling to the dog along its full length and to drape into the bun without sliding off in a clump. That texture is the engineering problem the sandwich solves: a chunky chili would fall out the ends on the first bite, so the Coney sauce is deliberately built to coat rather than to sit.
The craft is in the sequence and the bun. The frankfurter is the part everyone agrees on; it is there to be a firm, salty spine for a soft, wet load, and the natural-casing versions add a snap that the surrounding mush otherwise lacks. The bun is steamed rather than toasted, kept pillowy on purpose, because its job is to yield against the sauce, not to fight it. Raw onion supplies the only crunch and a sharp top note that cuts the rendered fat in the sauce; the mustard adds the acid that keeps the whole thing from reading as heavy. Built in order, dog then sauce then onion then mustard, it can be assembled in seconds at a counter and handed across without coming apart, which is the entire reason the format exists. Eat it standing up, fast, before the bun gives out.
The variations are a regional map drawn around the same loose-sauce idea. The Detroit Coney and the Michigan red and white sauces argue about spicing and consistency; the Cincinnati cheese Coney buries the sauce under a haystack of fine-shredded cheddar; the Flint version cooks a drier, almost ground-beef sauce that sits on top rather than soaking in. Each is a codified build with its own rules and its own town defending it, and each deserves its own article rather than being crowded in here.