The Sandwich Traiteur is the French sandwich at its most catered. It exists in the shop window of the traiteur, the prepared-food and cold-buffet specialist that occupies its own corner of the French food trade alongside the boulangerie and the charcuterie, and it shows up at apéro receptions, office events, weddings, and any gathering where a tray of crustless triangles needs to circulate. Its job is to be eaten with one hand while the other holds a glass of crémant.
The format is small, neat, and built for repetition. A loaf of pain de mie is sliced, layered with a single elegant filling, and trimmed of crusts before being cut into precise rectangles or triangles. Typical fillings include smoked salmon with crème fraîche and dill, jambon and butter, rillettes thinned to a spread, or thinly sliced cucumber with herbed cream cheese. The bread is soft and never toasted. The filling is generous but not sloppy. The cut edges are sharp. A plate of finished Sandwiches Traiteur reads as a single composition rather than a pile of food, which is exactly the visual effect the host is paying for.
Variations are organized by the apéro context rather than by ingredient. The cocktail format is small enough to be eaten in two bites. The buffet format is larger, more substantial, and shows up on platters covered with cellophane in office break rooms. The pain surprise tradition, a hollowed-out boule filled with assembled mini-sandwiches and presented as a single object, is the most theatrical cousin, and shows up at New Year's tables and certain regional weddings. The closely related catering tradition that uses puff pastry rather than soft bread is part of the broader French apéro repertoire but doesn't sit inside the sandwich family. See Pain Garni for the non-baguette bread traditions this format draws from.
The Sandwich Traiteur is not what a French person would order for lunch on their own. It belongs to a social context, an event, a tray, a moment of standing-and-conversing rather than sitting-and-eating. That use case shapes everything about it: the smallness, the precision, the soft bread, the cool filling. It is the form the French sandwich takes when its primary job is to be passed around.