Corned beef on rye is defined by the cure and the boil, not the assembly. Beef brisket is held in a salt brine until it takes on a clean, dense, faintly tangy character, then simmered until it slices tender and lean. It is piled hot onto two thin slices of rye with a swipe of mustard. The build is the same deliberate imbalance the deli runs everywhere: a heavy stack of meat against barely any bread, with the bread there to give the hands a grip and the mustard a place to sit, not to add bulk. What makes this its own sandwich rather than a pastrami is the brisket's leaner, brighter, boiled profile, where pastrami goes smoky and rich.
As a sandwich it works because every element is kept subordinate to the meat. The rye is a seeded, slightly sour loaf, used thin so it frames the corned beef without competing with it. The slicing is the load-bearing technique: hand-cut, against the grain, thick enough to have presence but tender enough to fold, because corned beef sliced machine-thin turns to thread and the stack falls apart in the hand. The meat is served hot so it drapes and compresses into something a hand can lift; the same brisket cold would sit stiff and shed slices. The mustard is the only seasoning the build adds, and it earns its place: sharp and acidic, it cuts a fattier slice and keeps a lean one from reading flat, balancing a sandwich that is otherwise one sustained savory note.
Corned beef on rye belongs to the Jewish deli's smoked-and-cured beef counter, where the close relations differ by cure and by what is layered on. Hot pastrami runs a peppered, smoked, steamed profile on the same rye. The Reuben griddles corned beef with Swiss, sauerkraut, and Russian dressing; the Rachel swaps the kraut for slaw and often the corned beef for turkey. Combination builds stack corned beef and pastrami together, and tongue and brisket round out the same case. Each of those is its own sandwich and deserves its own article rather than being crowded in here.