The trapizzino is a piece of bread engineered to be a bowl, and that is the whole idea before any filling enters it. A triangle of pizza bianca style dough is baked, then split along one edge and opened into a hollow pocket, and into that pocket goes a spoonful of a wet Roman braise or stew. The defining fact is that the dough has to do two opposite jobs at once: stay crisp and chewy enough to hold in the hand like bread, and stay sealed and sturdy enough to carry a loose, saucy, slow-cooked filling without going to paste or splitting at the seam. The dough is the vessel and the structure; the sugo or stew is the entire flavour and the reason to pick one up. Neither half is optional. A dry pocket with nothing in it is just a wedge of flatbread, and a ladle of Roman braise with no pocket is a plate of stew. The two are built to need each other, the bread as edible container and the wet filling as its only contents.
A good one starts with the dough and how it is handled. The base is a high-hydration pizza bianca dough, fermented slowly so it bakes up with an open chewy crumb and a thin crisp shell, baked as a flat triangle and then opened while still warm so the inside stays soft enough to hold a generous spoonful without cracking. The pocket is opened wide but not torn through, the closed point and two sides kept intact so the filling cannot run out the bottom. The filling is a properly cooked Roman braise or stew, reduced until it is thick and glossy rather than soupy, because a watery sauce will soak the crumb and a stiff dry one defeats the point of a sauce vessel. It is spooned in warm, enough to fill the pocket to the open edge without overflowing down the hand. The crisp shell on the outside is what keeps the whole thing holdable while the inner crumb soaks up just enough sauce to taste of it. A sloppy build uses underbaked dough that collapses, a thin sauce that leaks, or a pocket torn at the seam so the filling escapes on the first bite; a careful one bakes the dough to crisp-and-chewy, reduces the sugo to a thick coat, opens the pocket cleanly, and fills it to a level the bread can carry.
The close cousins are defined entirely by what goes in the pocket, and each deserves its own article rather than being crowded in here. Fill it with oxtail braised Roman style and it becomes the coda alla vaccinara trapizzino. Spoon in layered aubergine, tomato, and cheese and it is the parmigiana version. Use braised chicken with peppers and it is pollo alla cacciatora; use meatballs in tomato sugo and it is polpette al sugo; use slow-cooked tripe in its sauce and it is trippa alla romana. Every one of those is the same dough pocket carrying a different Roman braise, and the plain trapizzino, the engineered vessel itself, is the baseline they are all best read against first.